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Class 7~JM f 

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Copyright N° 



COPYRIGHT DEPOSIT 



Compressed Air Plant 
For Mines 



THE PRODUCTION, TRANSMISSION AND 

USE OF COMPRESSED AIR, WITH 

SPECIAL REFERENCE TO 

MINE SERVICE 



By ROBERT PEELE 

Mining Engineer and Professor of Mining in the School of Mines, Columbia University 



FIRST EDITION 

FIRST THOUSAND 



NEW YORK: 

JOHN WILEY & SONS 

London: CHAPMAN & HALL, Limited 
1908 



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Copyright, 1908, 
By ROBERT PEELE 



PRESS OF THE PUBLISHERS PRINTING COMPANY, NEW YORK, U. S. A. 



PREFACE 

The increasing use of compressed air makes the subject of 
interest to practitioners in nearly all branches of engineering. 
Besides its more important power applications, such as the 
operation of rock-drills, air brakes, riveting machines, and rail- 
road switching and signalling systems, the uses of compressed air 
are numerous in many minor branches of mechanical engineer- 
ing, in caisson work and the construction of subaqueous founda- 
tions, and in manufacturing industries, chemical works, etc., 
where it serves a multitude of purposes entirely distinct from that 
of the transmission of power. 

A realization of the breadth of the field has suggested that a 
book may be acceptable, addressed especially to those who are 
engaged in mining, tunnelling, quarrying, and other work in- 
volving the excavation of rock, with its concomitant operations. 
While the literature bearing upon this branch of compressed-air 
service is by no means small, it is for the most part scattered 
through the technical periodicals and transactions of engineering 
societies, and therefore not readily accessible to those who are 
out of convenient reach of engineering libraries. I am aware that 
little that is new can be said on this subject, and in writing the 
book I have availed myself freely of existing sources of informa- 
tion. 

In the first part, I have endeavored to present a view of cur- 
rent practice as to the construction and operation of compressors. 
Portions of the subject are dealt with at some length, such as: 
the types of compressor suitable for different kinds of service, heat 
losses occurring in air compression, and the various forms of 
valves, valve-motions, and governing and unloading mechanisms, 
that constitute prominent features of modern compressor practice. 

iii 



j v PREFACE 

A brief review is given of a few of the fundamental principles of 
air compression, but my intention has been to present only 
enough of the theory to make intelligible the formulas employed 
for the ordinary calculations of the power and capacity of com- 
pressed-air plant, together with the questions concerning tempera- 
ture changes, as affecting the production and use of compressed 
air. Many details of the design of compressors and proportions 
of their parts have been omitted, since these fall properly within 
the province of the mechanical engineer. The second part is 
devoted to the applications to mine service of compressed-air 
transmission of power; including machine drills, pumps operated 
by compressed air, and mine haulage by compressed-air locomo- 
tives. 

Many of the illustrations are reduced or adapted from work- 
ing drawings kindly supplied by compressor builders. Others 
have been taken from catalogues of compressed-air machinery 
and from technical periodicals and books dealing with the different 
types. The origin of these has been stated in nearly every in- 
stance. My thanks are due to several of the technical journals, 
especially Compressed Air Magazine and Mines and Minerals, 
for many suggestions and in some cases for passages extracted 
either in substance or verbatim, from articles therein contained. 
For any important use or adaptation of published material, 
permission has been asked and obtained, and frequent references 
are given in foot-notes or in the body of the text. I also wish to 
acknowledge my indebtedness to Mr. Frank Richards's book on 
" Compressed Air," from which I have derived substantial as- 
sistance. 



ROBERT PEELE. 



School of Mines, Columbia University, 
New York, May, 1908. 



CONTENTS 



PART FIRST 

PRODUCTION OF COMPRESSED AIR 

PAGE 

Preface, v 

List of Illustrations, xi 

CHAPTER I 

Introduction. Development of Air Compressors. Compressed Air 

versus Steam and Electric Transmission of Power, ..... i 

CHAPTER II 

Structure and Operation of Compressors : Straight-Line and Duplex. 
Compound Steam End; Stage Compressors; Direct- and Belt- 
Driven or Geared Compressors. Comparison of Types. Rela- 
tion of Work Done in Air and Steam Cylinders. Proportions of 
Cylinders. Compressors Driven by Water Power, ..... 8 

CHAPTER III 

The Compression of Air. Outline of the Theory. Operation of Air 
Compressors. Isothermal and Adiabatic Compression. Modes 
of Absorbing the Heat of Compression, 43 

CHAPTER IV 

Wet Compressors. Hydraulic-Plunger and Injection Compressors. 

Injection Apparatus. Quantity of Injection Water Required, . . 52 

CHAPTER V 

Dry Compressors. Construction of the Water-jackets. Circulation 
of Cooling Water. Piston Clearance and its Effect on Volumetric 
Capacity. Dry versus Wet Compression. Effect of Moisture 
in the Air under Compression. Effect of Injected Water, . . 58 



vi CONTENTS 



CHAPTER VI 



PAGE 



Compound or Stage Compressors. Theory and Operation. Single- 
and Double-Acting Stage Compressors. Construction and 
Functions of the Intercooler. Deductions from the Indicator 
Card of the Stage Compressor, 71 

CHAPTER VII 

Air Inlet Valves. Chief Requisites of . Poppet Inlet Valves: their 
Construction and Operation. "Skip-Valves" for the High- 
Pressure Cylinder of Stage Compressors. Ingersoll-Sergeant 
Piston-Inlet Valve. Johnson Valve. Humboldt Rubber Ring 
Valve. Leyner Flat Annular Valve. Arrangements for Ad- 
mitting Inlet Air to the Compressor, 91 

CHAPTER VIII 

Discharge or Delivery Valves. Spring-controlled Poppet Valves. 
Cataract-controlled Poppets. Riedler Discharge Valve. Dis- 
charge Area for Air Cylinders, ,110 

CHAPTER IX 

Mechanically Controlled Air Valves and Valve Motions. Mechanical 
Control for Discharge Valves. Norwalk, Nordberg, Laidlaw- 
Dunn-Gordon, Allis-Chalmers, Sullivan, Riedler, and Other 
Valve Motions. Cam-controlled Inlet Valve. Sturgeon Inlet 
Valve. Piston Valves, 1.16 

CHAPTER X 

Performance of Air Compressors. Standards of Rating. Calculation 
of Horse-Power of Single-Cylinder and Stage Compressors. 
Mean Cylinder Pressure. Temperature of Compression. Ele- 
ments of Air Indicator Card, 133 

CHAPTER XI 

Air Receivers. Construction and Functions. Underground Re- 
ceivers. Value of Cooling in the Receiver. " Receiver After- 
Coolers," 144 

CHAPTER XII 

Speed and Pressure Regulators for Compressors. Speed Governors. 
Air Cylinder Unloaders. Modes of Regulation for Steam- and 
Belt-Driven Compressors, 150 

CHAPTER XIII 

Air Compression at Altitudes above Sea-Level. Consequent Reduc- 
tion of Volumetric Capacity of Compressor. Relation between 
Compressor Output and Barometric Pressure. Mechanically 
Controlled Inlet Valves for High Altitudes. Stage Compression 
at High Altitudes, 164 



CONTENTS vii 



PAGE 

CHAPTER XIV 



Explosions in Compressors and Receivers. Discussion of Causes 
Heat of Compression. Cylinder Temperatures and Flashing- 
Points of Lubricating Oils. Examples of Explosions. Effect 
of Leakage of Delivery Valves. Precautions for Preventing 
Explosions, 17 1 

CHAPTER XV 

Air Compression by the Direct Action of Falling Water. Theory t 
of. Taylor Hydraulic Compressor. Descriptions of Plants at 
Magog, Province of Quebec, Ainsworth, B- C, and Victoria 
Copper Mine, Mich. Results of Tests, 183 



PART SECOND 

TRANSMISSION AND USE OF COMPRESSED AIR 

CHAPTER XVI 

Conveyance of Compressed Air in Pipes. Loss of Power. Loss of 
Pressure or Head. Discharge Capacity of Piping. D'Arcy's 
Formula. Richards's Formula for Loss of Pressure. Com- 
parison of Results of Current Formulas. Compressed-Air 
Piping. Effect of Bends in Pipe-Lines, 194 

CHAPTER XVII 

Compressed-Air Engines. General Considerations. Working at 
Full Pressure or with Partial or Complete Expansion. Ratios of 
Pressures and Temperatures due to Expansion in a Motor Cylin- 
der. Corrections for Piston Clearance, etc. Nominal and 
Actual Cut-off. Work Done in a Motor Cylinder. Volume of 
Free Air Required. Cummings "Two-Pipe" System, . . . , 205 

CHAPTER XVIII 

Fieezing of Moisture Deposited from Compressed Air. Causes and 
Prevention of Freezing. Influence upon Freezing of High Press- 
ures in Transmission. Deposition of Moisture by Reduction of 
Pressure. Protection of Air Piping, 220 

CHAPTER XIX 

Reheating Compressed Air. Appliances for, and Results of Reheat- 
ing. Temperatures Employed and Consumption of Fuel. 
Construction and Operation of Reheaters. Use of Reheaters for 
Underground Work. Wet and Dry Reheating, . . . . . .225 



viii CONTENTS 

PAGE 

CHAPTER XX 

Compressed- Air Rock-Drills General Considerations as to Efficiency. 
Consumption of Air: Normal and at Altitudes above Sea-Level. 
Factors Affecting Air Consumption: Kind of Work, Character 
of the Rock, and Physical Condition of Drill. Examples from 
Practice. Proper Air Pressure for Machine Drills. Valve 
Motions, 240 

CHAPTER XXI 

Operation of Mine Pumps by Compressed Air. Disadvantages of 
Using Ordinary Steam Pumps. Simple, Direct-Acting Pumps. 
Cylinder Dimensions of Simple Pumps. Volume of Air for 
Non-Expansive Working. Horse-Power. Regulation of Ini- 
tial Air Pressure. Prevention of Freezing of Moisture. Com- 
pressed-Air-Driven Compound Pumps : Discussion of Modes of 
Using the Air. Application of Reheating, 250 

CHAPTER XXII 

Pumping by the Direct Action of Compressed Air. Pneumatic- 
Displacement Pumps. Merrill, Latta-Martin, and Harris Dis- 
placement Pumps Pohle" Air-Lift Pump: Theory and Opera- 
tion. Tests on Air-Lift Pumps. Application for Pumping 
Slimes in South -African Mills. Lansell's Air-Lift for Pumping 
in Mine Shafts, 265 

CHAPTER XXIII 

Compressed-Air Haulage for Mines. Compressed Air versus Electric 
Locomotives. Construction and Operation of Compressed-Air 
Locomotives. Modes of Dealing with Low Cylinder Temperature. 
Calculation for Pipe-Line and Charging Stations. Charging 
Apparatus. Calculation of Motive Power. Compressors for 
Charging Pneumatic Locomotives. Detailed Examples of Com- 
pressed-Air Haulage Plants, 282 



ILLUSTRATIONS 

PAGE 

Figs, i and 2. — Laidlaw-Dunn-Gordon Straight-Line Compressor. Plan 

and Elevation, s ... 10, 11 

Fig. 3. — Ingersoll-Rand Straight -Line Compressor, Class A-i, ..... 13 

Figs. 4 and 5. — Laidlaw-Dunn-Gordon Duplex Compressor. Plan and Ele- 
vation, , 14, 15 

Fig. 6. — King-Riedler Compound Vertical Two-Stage Compressor, ... 16 

Fig. 7. — Norwalk Compound Straight-Line, Two-Stage Compressor. Longi- 
tudinal Section, ...... 18 

Fig. 8. — Norwalk Straight-Line, Two-Stage Compressor, with Simple Steam 

End, . . 19 

Figs. 9 and 10. — Leyner Straight-Line, Two-Stage Compressor. Plan and 

Elevation, 20 

Fig. 11. — Sullivan Straight-Line, Two-Stage Compressor. Longitudinal Sec- 
tion, Inset page 22 

Fig. 12. — Sullivan Duplex, Two-Stage Compressor. Longitudinal Section 

through Low-Pressure Cylinder, 21 

Fig. 13. — Leyner Duplex, Two-Stage Compressor, with Simple Steam Cylin- 
ders, 23 

Figs. 14 and 15. — Riedler Cross-Compound Two-Stage Compressor; 15" and 

24"X 36" Air Cylinders. Plan and Elevation, 24,25 

Figs. 16 and 17. — Allis-Chalmers Cross-Compound Corliss, Two-Stage Com- 
pressor. Plan and Elevation, 26, 27 

Figs. 18 and 19. — Laidlaw-Dunn-Gordon Duplex, Cross-Compound Compres- 
sor, with Two-Stage Air Cylinders. Perspective View and General 
Plan, Elevations and Sections, Inset and page 29 

Fig. 20. — Combined Air and Steam Cards, 31 

Fig. 21. — Duplex, i6"X3o" Risdon Compresser, Driven by 16 ft. Water-wheel, ^^ 

Figs. 22 and 23. — Water-Driven Risdon Duplex Compressor. Plan and Ele- 
vation, 34, 35 

Fig. 24. — Ingersoll-Rand Water-Driven Compressor, 37 

ix 



ILLUSTRATIONS 



PAGE 

Figs. 25 and 26. — Rix Water-Driven Compressor at North Star Gold Mine, 

Calif. Side and Front Elevations, 38, 39 

Fig. 27. — Ingersoll-Sergeant Straight-Line, Belt -Driven Compressor, ... 41 

Fig. 28. — Air Compression Temperature Diagram, 47 

Fig. 29. — Air Indicator Card, 49 



Figs. 30, 31 and 32. — Air Indicator Cards, Showing Effect of Cooling, 
Fig. t,^. — Humboldt Wet Compressor, 



5 1 

53 
54 

59 
60 



Fig. 34. — Hanarte Wet Compressor, . . 

Fig. 35. — Air Cylinder of Nordberg Compressor, ......... 

Fig. 36. — Air Cylinder, Class E, Laidlaw-Dunn-Gordon Co., .... 

Fig. 37. — Air Card Showing Effect of Clearance, 63 

Fig. 38. — Diagram of Effect of Clearance on Capacity of Dry Compressor, . 64 

Fig. 39. — Section of Air Cylinder, Showing Method of Reducing Clearance, . 65 

Fig. 40. — Section of Piston, Johnson Compressor, 65 

Fig. 41. — Diagram of Norwalk Two-Stage Compressor, ....... 76 

Fig. 42. — Horizontal Intercooler. Ingersoll-Rand Co., 81 

Fig. 43. — Intercooler. Sullivan Machinery Co., .......... 82 

Fig. 44. — Leyner System of Intercooling, . „ 85 

Fig. 45. — Vertical Intercooler. Ingersoll-Rand Co., 87 

Fig. 46. — Combined Air Card of Two-Stage Compressor, ....... 88 

Fig. 47. — Norwalk Poppet Inlet Valve, 94 

Fig. 48. — Laidlaw-Dunn-Gordon Poppet Inlet Valve, 95 

Fig. 49. — Diagram of Effect of Valve-Spring Resistance on Volumetric Capac- 
ity of Compressors, 97 

Fig. 50. — Air Card Showing Effect of Valve Resistance, 9§ 

Fig. 51. — "Skip-Valve." Norwalk Iron Works Co., 100 

Fig. 52. — Cylinder of Piston-Inlet Compressor. Ingersoll-Rand Co., . . 101 

Figs. 53 and 54. — Johnson Air Valves, 103 

Fig. 55. — Humboldt Rubber Ring Valves, 104 

Fig. 56. — Leyner Compressor. Part Section, Showing Flat Annular Air 

Valves, 106 

Fig. 57. — Leyner Annular Inlet Valve, 107 

Fig. 58. — Laidlaw-Dunn-Gordon Poppet Discharge Valve, in 

Fig. 59. — Norwalk Poppet Discharge Valve, 112 



ILLUSTRATIONS xi 

PAGE 

Fig. 60. — "Express" Poppet Valve. Riedler Compressor, 113 

Fig. 61. — Valve Motion of Low-Pressure Air Cylinder. Norwalk Com- 
pressor, 119 

Fig. 62. — Section of Air Cylinder of Nordberg Compressor, 120 

Fig. 63. — Section of Air Cylinder. Laidlaw-Dunn-Gordon Co., 121 

Fig. 64. — " Cincinnati " Valve Gear. Laidlaw-Dunn-Gordon Compressor, . 122 

Fig. 65. — Standard Air-Valve Motion. Allis-Chalmers Co., 124 

Fig. 66. — Sullivan Air Cylinder, Showing Corliss Inlet Valves, 125 

Fig. 67. — Riedler Air- Valve Motion, 127 

Fig. 68. — Details of Riedler Inlet Valve, 128 

Fig. 69. — Details of Riedler Discharge Valve, ........... 129 

Fig. 70. — Cam-Controlled Inlet Valve, 130 

Fig. 71. — Sturgeon Inlet Valve, 131 

Fig. 72. — Ideal Air Card, 141 

Fig. 73. — Diagram. Elements of Air Indicator Card, 141 

Fig. 74. — Vertical Air Receiver. Norwalk Iron Works Co., 145 

Fig. 75. — Horizontal Receiver-Aftercooler. Ingersoll-Rand Co., . . . 146 

Fig. 76. — Clayton Governor and Pressure Regulator, 151 

Fig. 77. — Norwalk Pressure Regulator, 153 

Fig. 78. — Norwalk Pressure Regulator, 154 

Fig. 79. — Clayton Pressure Regulator, 155 

FlG. 80. — Rand Imperial Unloader. Sectional View, 157 

FlG. 81. — Sullivan Governor and Unloader, 158 

Fig. 82. — Ingersoll-Sergeant Regulator and Unloader, 160 

Fig. 83. — Laidlaw-Dunn-Gordon Air Governor, 161 

Fig. 84. — Air Cards Showing Results of Compression at Altitudes above 

Sea Level, 165 

FlG. 85. — Taylor Hydraulic Air Compressor, 185 

FlG. 86. — Taylor Hydraulic Air Compressor. Detail of Head-piece, . . . 186 

Figs. 87 and 88. — Hydraulic Air-Compressing Plant at Kootenay, B. C, 

o . . . . . . . o . . . „ Inset and page 190 

Fig. 89. — Expansion Curves of Steam and Air, „ . 209 

Fig. 90. — Card Showing Work Done in Motor Cylinder, . . . . . . .213 

Fig. 91. — Leyner Compressed-Air Reheater, . .... 231 



Fig. 


93- 


Fig. 


94. 


Fig. 


95- 


Fig. 


96. 


Fig. 


97- 


Fig. 


98. 


Fig. 


99. 



Xll ILLUSTRATIONS 

PAGE 

Fig. 92. — Cast-iron Coils, Leyner Reheater, 232 

-Sergeant Reheater, 233 

. — Rand Reheater, 234 

. — Sullivan Reheater, 235 

-Merrill Pneumatic Pump, 266 

. — Diagram of Pohle Air-Lift Pump, 271 

-Diagram of Lansell's Air-Lift Pump for Mine Shafts, .... 280 

-H. K. Porter Four-Wheel, Single-Tank Compressed-Air Mine Lo- 
comotive, 286 

Fig. 100. — Small H. K. Porter Compressed-Air Locomotive, 287 

Fig. ioi. — Baldwin Six-Wheel Compressed-Air Locomotive, 288 

Fig. 102. — Baldwin Four-Wheel Compressed-Air Locomotive, 288 

Figs. 103, 104 and 105. — Plan, Elevations and Sections of Baldwin Com- 

pressed-Air Locomotive, 289, 290, 291 

Fig. 106. — Compressed-Air Locomotive Charging-Station, 297 

Fig. 107. — Norwalk Locomotive Charging Compressor, 301 

Fig. 108. — Air-End of Ingersoll-Rand Three-Stage Locomotive Charger, . . 302 

Figs. 109 and no. — Low- and High-Pressure Air-Ends of Ingersoll-Rand 

Four-Stage Compressor, 303 

Fig. hi. — Perspective View of Ingersoll-Rand Four-Stage Compressor, . . 304 

Fig. 112. — E. A. Rix Compressed-Air Locomotive for Empire Mine, Grass 

Valley, Cal., 307 



COMPRESSED AIR PLANT FOR MINES 



Part First 
PRODUCTION OF COMPRESSED AIR 



CHAPTER I 

INTRODUCTION 

One of the most important applications of the transmission of 
power by compressed air is the driving of machine rock-drills; 
and to the necessity of providing for these drills a power medium 
suitable for use in mines and tunnels has been due, more than to 
any other cause, the development of the modern air compressor. 

The time which has elapsed since the beginnings of this branch 
of engineering is short. The first percussion rock-drill, operating 
independently of gravity, was invented in 1849 by J- J- Couch, of 
Philadelphia. Though used only experimentally, it embodied the 
principal mechanical features of the modern machine drills, which 
have had such a striking influence in mining and tunnelling. 
Couch's machine, together with its immediate successors, such 
as the Fowle drill (1849-51) and the Cave (Paris, 1851), 
were steam-driven and therefore unsuitable for underground 
work. In 1852, the physicist Colladon proposed the use of com- 
pressed air for operating rock-drills, in connection with the driving 
of the Mont Cenis tunnel, in the western Alps. His idea was de- 
veloped by Sommeiller and others between 1852 and i860, and in 
1861-62 an air-compressor plant was first used successfully at that 

l 



2 COMPRESSED AIR PLANT TOR MIXES 

tunnel. It was driven by water power and furnished air for 
ventilation as well as for the drills. 

The transmission of power by compressed air thus dates from 
about the middle of the last century. It is hardly necessary to 
say that the early air compressors were crude in both design and 
construction. Sommeiller's first plant, though of large size and 
effectual in fulfilling its purpose, had some resemblance in principle 
to the old hydraulic ram, possessing no moving parts except the 
valves. Piston compressors, driven by steam engines, such as the 
Dubois-Francois, and more or less similar fundamentally to some 
of the wet compressors still in use, soon made their appearance. 
Probably the first compressors built in the United States were 
those employed at the Hoosac tunnel, in western Massachusetts, 
in 1865-66. The Burleigh, Xorwalk, Clayton, and Rand compres- 
sors were among the earlier makes in this country. 

But the Mont Cenis tunnel, about eight miles long and com- 
pleted in 1 87 1, the first connecting link through the Alps be- 
tween the railway systems of France and Italy, was undoubtedly 
the field where were fought out on a large scale the initial problems 
of compressed-air production and use; and to Sommeiller is due the 
honor of having laid the foundations of new practice, by which that 
great work was brought to a successful completion. From 1857 
to 1 861 the tunnel headings had been progressing slowly and in 
the face of great difficulties. Drilling was done by hand labor 
and blasting by black powder, the average advance for this period, 
in each of the two headings, being only about one and a half 
feet per day. At this rate, even granting that the work could 
have been finished at all by the means employed, over forty 
years would have been required to connect the headings and 
years more to complete the enlargement to full section. With 
machine drills, the speed of advance in each heading rose to four 
and three-quarters feet per twenty-four hours and later, when 
dynamite was introduced, to a little over six feet; this average 
being maintained for a period of six years. 

Machine drills did not make their way into mining to any extent 
for some years after their successful application to tunnel driving. 



INTRODUCTION 3 

It is difficult now to name the mining district in this country where 
they were first used, but their most important trial was probably 
in the Calumet and Hecla copper mine, Michigan. After strong 
and concerted opposition from the miners, the Rand drill was 
introduced there in 1878, and the value of machine drilling for 
hard ground was soon demonstrated by decreased costs of drifting 
and stoping and higher speeds of advance. 

Compressed air has now a wide application in various branches 
of mechanical engineering and the arts and manufactures. In this 
book it is intended to deal only with its production and uses in 
connection with mining and tunnelling operations. Its two rivals 
in these fields of work are steam and electricity, regarding which 
a few general considerations may here be mentioned. 

As compared with steam, compressed-air transmission of pow- 
er is especially valuable and convenient for three reasons: first, its 
loss in transmission through pipes is relatively small; second, the 
troublesome question of the disposal of exhaust steam underground 
is avoided ; third, the exhausted air is of some assistance in ventilat- 
ing the working places of the mine. In large mines, where steam 
may be carried thousands of feet, down shafts and through lateral 
workings, for operating pumping engines, etc., the disadvantages 
attending its use become very apparent; the amount of condensa- 
tion is serious, even when the piping is provided with good non- 
conducting covering, and the working efficiency falls to an abnor- 
mally small figure. Furthermore, aside from the heat produced 
by the use of steam, it is rarely feasible to employ efficient con- 
densers for underground engines other than pumps, on account of 
the difficulty of obtaining the necessary condensing water and the 
additional space required. If the exhaust be discharged into the 
mine workings, even though they are large and well ventilated 
and the volume of the exhaust steam comparatively small, the 
temperature and quantity of moisture in the air would be con- 
siderably increased. Deterioration of the timbering is thereby 
hastened, the roof and walls of the workings are softened and 
slacked off, especially in collieries, and the mine atmosphere is 
rendered oppressive and unwholesome. The presence of hot steam 



4 COMPRESSED AIR PLANT FOR MINES 

pipes in confined workings, or in the narrow compartments of 
shafts, is also objectionable. 

Although the loss from condensation in long steam lines may 
be diminished by covering the pipe with efficient non-conducting 
material, still, even with the best covering, the effective pressure at a 
distant underground engine is greatly reduced, and very uneconomi- 
cal working is the result. On conveying steam a distance of several 
thousand feet the pressure may be reduced to half the boiler press- 
ure, or even less. For example, in the case of a pump, or other 
engine, situated 2,000 feet from the boiler and using 200 cubic feet 
of steam per minute at a boiler pressure of 75 pounds, with a 
four-inch mineral-wool-covered pipe, the effective pressure at the 
engine would be only about 58 pounds; or, with a poor covering, 
like some of the asbestos lagging often used, it might easily be as 
low as 35 pounds. For compressed-air transmission, on the other 
hand, the reduction of pressure for the same volume of air, size of 
pipe, and initial pressure, would be 9.3 pounds, giving a terminal 
pressure of 65.7 pounds. However, as the speed of flow in pipes 
for economical transmission is greater for steam than for air, a 
comparison based solely on piping of the same diameter cannot 
justly be made. In the above example, if the diameter of the pipe 
were smaller the gain in reduced radiation would outweigh the in- 
creased f rictional loss, and the net loss would be diminished. Since 
the frictional loss varies inversely, and the loss from radiation 
directly, with the diameter, the size of the steam pipe can be so 
proportioned as to produce a minimum loss under the given con- 
ditions. With compressed air the case is different, since the ques- 
tion of radiation is eliminated. If the diameter of the pipe be 
increased to 5 inches the loss of pressure, or head required to over- 
come friction, is reduced to 2.8 pounds and increasing the distance 
to one mile it would be only 7.4 pounds. Furthermore, the in- 
creased cost of the larger air pipe would be offset by the expense 
of the non-conducting covering necessary for steam transmission. 

Thus, compressed air may be conveyed long distances with but 
small loss of pressure, and is readily distributed for application to a 
variety of underground uses, for which steam is not practicable. 



INTRODUCTION 



Compressed air is always ready to do its work, and, aside from 
leakage of transmission pipes, which is in large measure pre- 
ventable, suffers no loss nor diminution of power when not in 
actual use. For performing work intermittently, at a distance from 
its source, it is therefore particularly valuable, because the air press- 
ure is maintained nearly constant during intervals of work, with- 
out further expenditure of power. With steam transmission, on 
the contrary, power is continually dissipated by radiation, whether 
in use or net, and a steam engine, when stopped for any length of 
time, loses much of its normal working temperature and becomes 
a receptacle for water of condensation. 

Though in mining compressed air is employed mainly for oper- 
ating machine drills, other applications are found in the driving of 
underground hoists and pumps in confined workings. Mechanical 
coal cutters, for mining bituminous coal, are sometimes operated 
by compressed air, and the employment of compressed-air loco- 
motives in mines and extensive tunnelling operations furnishes an 
example of its capacity for storing power, in contradistinction to its 
function as a power transmitter. The introduction of compressed- 
air drills has facilitated the rapid driving of long railroad and 
mining tunnels, which otherwise would have been greatly delayed 
or completed only with extreme difficulty. Had compressed-air 
power, together with the high explosives, not been available, it may 
well be doubted whether the great tunnels through the Alps and 
elsewhere, and the numerous long mine tunnels driven in recent 
years in this country, would have been at all practicable. 

Without attempting to review in detail the comparative merits 
of electricity and compressed air, it may be pointed out that the ap- 
plication of electricity for transmitting power in mines has increased 
enormously in importance during the past twenty-five years. The 
peculiar requirements of mine service have been in nearly all cases 
successfully met by modifications and adaptations of standard forms 
of electric apparatus. Both means of power transmission possess 
characteristics which adapt them particularly for underground 
work. But, although by virtue of its numerous successful applica- 
tions, electricity has become a rival of compressed air in most 



6 COMPRESSED AIR PLANT FOR MIXES 

branches of mine work, their spheres of usefulness are not identical 
and the field is broad enough for both. It is often stated that the 
first cost of an electric plant is lower than that of an equivalent com- 
pressed-air plant. A broad generalization, however, does not cover 
the case. There is actually but little difference between the costs 
of the power plants themselves, the advantage being generally 
with the compressor. Considering the question of the transmission 
of a given power, the cost of the electric conductor line for short dis- 
tances is much less than that of compressed-air pipe; but the cost 
of the electric line increases as the square of the distance, while the 
cost of the pipe line increases only as the first power of the distance. 
Hence, a point is soon reached where compressed-air transmission 
becomes the cheaper. It is in the greater efficiency of generation 
that electric power has the advantage. 

In one direction only has electricity failed hitherto to meet even- 
requirement. While compressed-air drills, though far from being 
economical considered simply as machines, nevertheless admirably 
fulfil their purpose, no perfectly satisfactory electric rock-drill has 
yet been produced. However, this problem has for years been 
receiving much attention from electricians, both in this country and 
abroad, and there is reason to anticipate a successful solution in the 
near future. The Temple " electric-air " drill, brought out some 
four years ago, and already well tested under a variety of conditions, 
mav be referred to here as a remarkably efficient and ingenious 
machine, but it is not an electric drill in the ordinary meaning of 
the term. It is rather a combination of a compressed-air drill, 
operated by a small, electric-driven compressor which is mounted 
on a truck close to the drill itself. As there is no exhaust, the same 
air being used over and over, one of the incidental advantages of the 
ordinary air drill is missing, viz., that of assisting somewhat in 
ventilating the mine workings, in those places where ventilation is 
most needed. Keeping this in mind, together with such minor 
uses of compressed air as the cleaning of drill holes preparatory to 
charging, and driving out the smoke of blasting from working 
places, it seems doubtful whether, for underground mining, electric 
drills of any kind can be expected to supersede entirely those oper- 



INTRODUCTION 7 

ated by compressed air. Given the necessity for a compressed-air 
plant for the rock-drills, as is the case in most metal mines, it may 
often be more advantageous to provide the additional compressor 
capacity required for driving underground pumps, hoists, and other 
machines as well, than to erect a separate and distinct plant for 
generating electricity. 

Because of the view usually taken of the lack of economy 
in the operation of compressed-air drills, it has been customary in 
the past to consider compressed air in general as a form of power 
respecting which the questions of convenience and expediency are 
more weighty than the attainment of a high degree of efficiency. 
In recent years, however, as the principles of air compression have 
become better understood, a substantial improvement has taken 
place, not only in the design of the compressors themselves, but also 
in the installation of pipe lines and in the operation of the machines 
using the compressed air. The consequences of overloading a 
compressor, and thereby driving it beyond its proper speed, are now 
comprehended by every intelligent master mechanic as being wholly 
different from those produced by overloading a steam engine. 
The results of leaks in air pipes, and of using air mains of too small 
a diameter, are also understood and avoided. Better practice pre- 
vails in the field, and in the production, transmission, and use of 
compressed air a much higher total efficiency is now realized than 
was formerly thought possible. 



CHAPTER II 

STRUCTURE AND OPERATION OF COMPRESSORS 

An air compressor consists essentially of a cylinder in which 
atmospheric air is compressed by a piston, the power for driving 
which may be derived from a steam engine, water-wheel, or electric 
motor. The air cylinder is almost invariably double-acting, and 
as such is provided with inlet and discharge or deliver}' valves in 
each cylinder head. On the forward stroke the air is compressed 
by the advancing piston, while the decrease in pressure, or, as it is 
commonly termed, the tendency to form a vacuum, behind the pis- 
ton causes the inlet valves to open under atmospheric pressure, thus 
allowing the outside air to flow into the cylinder. At each stroke 
a certain volume of compressed air is forced from the cylinder 
through the discharge valves, into a pipe leading to a large reservoir 
or receiver, whence the air enters the transmission pipe or main. 

Before considering the operation and various appurtenances of 
the air and steam cylinders, it will be well to examine the general 
mechanical structure of the compressor and the modes of applying 
the power. Probably no single classification of air compressors 
can be made sufficiently comprehensive to present intelligibly all of 
their salient features. In attempting a classification three widely 
different bases of comparison suggest themselves. First, several 
clear distinctions result from a consideration of the general struc- 
tural characteristics of air compressors regarded purely as engines; 
second, they may be classed according to the mode of dealing with 
the heat necessarily produced during compression of the air; and 
third, the numerous and varying types of valves and valve-motions 
devised in modern practice for controlling the distribution of the 
air in the compressing cylinders constitute a basis for comparison 

8 



STRUCTURE AND OPERATION OF COMPRESSORS 9 

which, though not so simple as the others, is in some respects 
quite as useful and important. 

The first classification only will be given here, the others being 
taken up respectively in Chapters IV to VI and VII to IX. Air- 
brake and gas compressors, vacuum pumps and other special forms 
of air-compressing machinery are not included, as this book will 
not deal with compressors other than those which are applicable to 
mine service. 

Under the first classification and taking the steam-driven com- 
pressor as the type form, four subdivisions may be named : 

1. " Straight-line' * Compressors. In these, which are made by 
all builders, the steam and air cylinders are set tandem on a common 
piston-rod. They are always provided with a pair of fly-wheels, 
one on each end of the crank-shaft, which are driven by outside 
connecting-rods from a cross-head between the steam and air 
cylinders. Their structural form is thus simple and compact to a 
marked degree. Figs. 1 and 2 illustrate a Laidlaw-Dunn-Gordon 
straight-line compressor, with Meyer valve gear, a section of the 
steam cylinder being shown in the plan and of the air cylinder in 
the elevation. Fig. 3 is a perspective view of an Ingersoll-Rand 
straight -line compressor. 

2. Duplex Compressors, (a) Two engines are placed side by 
side, each being complete in itself and consisting of tandem steam 
and air cylinders, with their cranks set at 90 degrees on a common 
fly-wheel shaft. Each side of the duplex is in effect a straight-line 
compressor. They are almost invariably horizontal, and the steam 
cylinders are always nearest the crank-shaft. Figs. 4 and 5 show 
a type of the duplex compressor, (b) One air and one steam cylin- 
der, side by side, with a common crank-shaft, may in one sense 
be classed as duplex, though its operation is entirely different 
from (a) in the disadvantageous distribution of the load. While 
obsolete in America, this form is occasionally adopted by some 
European builders for special purposes. It is not well balanced, 
occupies at least three times the floor space of a straight-line com- 
pressor of the same capacity, and requires more expensive founda- 
tions. 



IO 



COMPRESSED AIR PLANT FOR MINES 



3. Compressors with 
Compound Steam Ends. 

(a) Duplex, horizontal, 
cross-compound; a simple 
or single-stage air cylinder 
being set tandem to each 
steam cylinder. This form 
is now rarely used. The 
considerations leading to 
the compounding of the 
steam end make it desirable 
to adopt stage compression 
for the air end. (b) Verti- 
cal compound; the air cyl- 
inders being placed respec- 
tively above the high- and 
low-pressure steam cylin- 
ders. This also is a some- 
what unusual design. It is 
advantageous in saving 
floor space, though this con- 
sideration is rarely of con- 
sequence at mines. As an 
example, the King-Riedler 
compressor may be cited 
(Fig. 6).* Some very large 
plants of this type, up to a 
capacity of 8,oco cu. ft. of 
free air per minute, have 
been built for South Afri- 
can mines. Vertical com- 
pressors have the disadvan- 
tage of being complicated 
and difficult to maintain 
and repair. 

* From American Machinist, 
Oct. 16th, 1902, p. 1,475. 




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12 COMPRESSED AIE PLANT FOR MIXES 

4. Compound or Stage Compressors, in which the air cylinders 
themselves are compounded. The air end may be of the double-, 
triple-, or quadruple-stage type, according to the air pressure to be 
produced.* Stage compressors are now made by nearly all 
builders in the United States, and compose the most important class 
of air-compressors for general use. (a) Straight-line form, as in 
(1). These have two-stage air ends, some having compound steam 
ends also. Fig. 7 shows the longitudinal section of a Xorwalk 
compressor, with compound steam cylinders and Figs. 8, 9, 10 and 
11, respectively Xorwalk, Leyner, and Sullivan compressors, with 
simple steam cylinders, (b) Duplex steam end, with two-stage 
air cylinders. A longitudinal section of a Sullivan compressor 
of this class is given in Fig. 12, and a perspective view of a recent 
type of Leyner compressor in Fig. 13. (r) Duplex, cross-com- 
pound steam end, with two- to four-stage tandem air cylinders. 
These are designed for large plants only, and are widely used. 
When provided with air cylinders of more than two stages they are 
intended for special high-pressure sen-ice, such as furnishing air 
for underground compressed-air locomotives. Figs. 14 and 15 
show the general plan and side elevation of a Riedler, and Figs. 
16 and 17 similar views of an Allis- Chalmers Corliss compressor 
of this class; Fig. 18 is a perspective view, and Fig. 19 a repro- 
duction of a working drawing, in plan and elevations, of a Laidlaw- 
Dunn- Gordon compressor, which will further illustrate this type. 

As based on structural characteristics, compressors may also be 
classified as: (a) Direct-driven by steam- or water-power — the 
motor end being directly connected with the air cylinders. Among 
water motors the bucket or impulse wheels are best adapted to this 
service; (b) Belt-driven from independent motors: steam-engines, 
water-wheels, or electric motors. These are built by most of the 
American makers, and are in common use for mine and other 
sen-ice. Chain-driven and direct-geared compressors are also 
occasionally employed, as noted hereafter. 

*It may be noted that the Norwalk Iron Works Co. was the pioneer in the 
field of stage compression, having begun in 1880-81 to build this type of com- 
pressor for ordinary sen-ice. 




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So-called "half -duplex" compressors are furnished when re- 
quired. They consist of either the right- or left-hand half of a du- 
plex compressor, an extended crank-shaft and out-board pillow- 




Fig. 6. — King-Rjedler Compound Vertical Two-Stage Compressor. 



STRUCTURE AND OPERATION OF COMPRESSORS 1 7 

block being provided temporarily. An advantage of this form is 
that, if only a comparatively small quantity of air is needed for a 
time — as during the development of a mine or the sinking of a 
shaft — one-half of a duplex compressor may be installed at 
first, the second half being readily added when required. The 
capacity is thus doubled at a moderate cost. 

Comparison of Types of Compressor 

The straight-line compressor is largely employed for rather small 
plants or for temporary service. It is compact, strong, and self- 
contained, the entire engine being carried by a single bed-frame 
and requiring a relatively inexpensive foundation. The floor 
space occupied is much less than for the duplex form. The air 
and steam cylinders are just far enough apart to allow the cross-head 
and guides to be placed between them. From the cross-head the 
fly-wheels are driven by connecting-rods on each side. By using a 
pair of fly-wheels each is made smaller and lighter than if there were 
but one, and the moving parts are better balanced. While useful 
for moderate air pressures and fairly constant loads, and satis- 
factorily filling an important field of work ; the straight-line com- 
pressor is not capable of operating with the steam economy de- 
sirable and even essential in plants of large capacity; nor is it 
self -regulating at much less than, say, forty per cent, of its full load. 
These compressors are usually made of capacities from the smallest 
up to 1,700 or 1,800 cu. ft. of free air per minute, the last-named 
sizes developing from 275 to 300 horse-power. Further details of 
the operation and distribution of load in these compressors are given 
on page 28. 

The duplex compressor is always preferable to the straight-line 
for large plants. It is better adapted to varying loads, arising 
from differences of air pressure, because the resistance is more 
uniformly distributed throughout the stroke. By reason of its 
quartering cranks it may be run at extremely slow speeds without 
stopping on a center; and it is self -regulating and capable of deal- 
ing economically with a range of load down to considerably less 
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19 



than one-quarter or one-third of its normal. As a rule, the friction 
loss (total horse-power consumed by friction of the engine) of the 
duplex compressor is no greater and is often less than that of a 
straight-line of the same capacity. For large Corliss compressors, 
in good order, this loss may be put at not over five to seven per 
cent.* While these figures are sometimes equalled by the best 




Fig. 8. — Norwalk Straight-Line, Two-Stage Compressor, with Simple Steam End. 

straight-line compressors, it is safe to say that the loss in the latter 
is generally higher. 

Of late years the Corliss type of engine has come into general 
use for driving large duplex compressors, especially when com- 
pounded in both steam and air end, as its valve gear is well adapted 
for dealing with the variations of air pressure under which com- 
pressors are usually called on to work. By the majority of builders 
the Corliss valve gear is employed, at least for large plants, for the 
air as well as the steam cylinders. 

The foundation of the duplex compressor is necessarily more 

* In this connection, see an article by J. Parke Channing, in Mines and Miner- 
als, May, 1905, p. 475, containing the results of an efficiency test on a 300-H.-P. 
compound, two-stage Nordberg Corliss compressor, at the Burra-Burra mine of 
the Tennessee Copper Co. Its efficiency was found to be 78.1 per cent, total. 
The horse-power consumed by friction was only 5.2 per cent. 



20 



COMPRESSED AIR PLAXT FOR MIXES 




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22 COMPRESSED AIR PLANT FOR MIXES 

expensive than that of the straight-line, and must be substantially 
built if perfect alignment is to be maintained. Each pair of cylin- 
ders are solidly connected, either by trunk-frames or heavy tie- 
bolts. A complete girder-frame may be provided (Figs. io, 12) to 
avoid any possibility of movement. The tandem steam and air 
cylinders on each side are best placed far enough apart to prevent 
the same portion of the piston-rod from passing alternately into 
each stuffing-box. The reasons for this are: first, the piston-rod 
is apt to wear differently in the two stuffing-boxes, so that it becomes 
difficult to keep them well packed and tight; second, in this con- 
struction the steam and air piston-rods are made in separate parts, 
coupled together between the cylinders. This is a matter of con- 
venience in making repairs, when it becomes necessary to take the 
compressor to pieces; also, the air valves, when of the poppet 
form and in the cylinder head, are more accessible. An incidental 
advantage of the duplex compressor is that, as each half is com- 
plete in itself, one side may be disconnected for repairs or when a 
smaller capacity is temporarily desired. 

Compressors with Compound Steam Cylinders. The advantages 
in point of economy secured by compounding the steam end of air 
compressors are even more striking than in the case of ordinary 
stationary engines, for two reasons : First, because the conversion 
of power from one form to another is necessarily attended by some 
loss, and should therefore be conducted as economically as possible; 
second, because, as will be shown hereafter, the operation of 
compressing air involves particularly unfavorable load conditions. 
The valuable features of the duplex compressor become most 
apparent when the steam cylinders are compounded and furnished 
with a proper condenser. In plants of any size, a steam saving of, 
say, twenty percent, may thus be readily attained, not only by getting 
the full expansive power out of the steam, but also by avoiding 
frequent loss of power due to imperfect speed regulation and con- 
sequent blowing off of air at the relief or safety valve. 

Stage Compressors in recent years have come into general use 
for mining and other service. It is now recognized that even for 
ordinary pressures of, say, seventy-five pounds, such as are com- 




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28 COMPRESSED AIR PLANT FOR MINES 

monly employed for machine drills, a saving in steam consumption 
can be realized. In elevated mountain regions, where so much min- 
ing is carried on, the advantages of stage compression are still greater 
than at sea-level, as is shown in Chapter XIII. The duplex form, 
with both steam and air ends compounded, exemplifies the highest 
type of compressor. There is no material increase in the number 
of moving parts, except valves; the greatest range of steam ex- 
pansion is obtainable, because the work done in the air cylinders 
is more nearly equalized, and the compressor may be made self- 
regulating over its entire range of load. Thermodynamically, the 
efficiency of stage compression depends largely on the proper use 
of water-jackets for the cylinders, and the size and design of the 
intercooling apparatus between the air cylinders; a subject much 
better understood now than formerly. Stage compression is dis- 
cussed in detail in Chapter VI. 

Operation of Steam-driven Compressors. A steam-driven air 
compressor operates under peculiar conditions; appearing to 
work under a disadvantage which does not obtain in ordinary 
steam engines. This will be understood by inspecting the com- 
bined air and steam indicator cards of a simple straight-line com- 
pressor (Fig. 20). At the beginning of the stroke the air in front 
of the piston is at atmospheric pressure. As the piston advances 
the pressure at first increases slowly, while toward the end of the 
stroke it rises very rapidly. In other words, the resistance in the 
air cylinder varies from zero at the beginning of the stroke to its 
maximum near the end. The power developed in the steam cylin- 
der, on the contrary, when working as usual with a cut-off, is in 
exactly the reverse order. The initial steam pressure may be even 
lower than the final air pressure, though the mean effective pressure 
in the steam cylinder is greater than the mean effective in the air 
cylinder, as shown by the diagram. For example, with an initial 
steam pressure of sixty pounds, air may be compressed to eighty 
pounds or more. This result is obtained by the use of heavy fly- 
wheels and reciprocating parts, for carrying the engine over its cen- 
ters, storing up the surplus power in the early part of the stroke, and 
giving it out toward the end. It follows that there is a marked want 




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30 COMPRESSED AIR PLANT FOR MINES 

of smoothness in the running of compressors, which causes severe 
strains in the moving parts. This is specially noticeable in the 
simple straight-line type, which, when the air in the receiver is up 
to gauge pressure, will often be brought almost to a standstill and 
barely turn over the centers. It would thus appear that only a 
small ratio of expansion in the steam cylinder could be employed, 
and in fact some of the older forms of straight-line compressors 
took steam throughout nearly the entire stroke. But the difficulty 
is met, and greater economy made possible, by the inertia of the 
fly-wheels. The dimensions of the steam and air cylinders in 
simple compressors are proportioned for a cut-off of from | to J 
stroke. 

In most of the simple straight-line compressors the steam cylin- 
der is provided with an adjustable cut-off valve (Fig. i ). This valve 
a is composed of two parts and, moving on top of the main valve, 
controls ports in the latter through which steam is admitted to the 
main ports. It is operated by a separate eccentric on the fly- 
wheel shaft, and by means of the hand-wheel b, outside of the end 
of the valve chest, may readily be regulated without stopping the 
compressor, according to the varying pressure in the receiver. By 
manipulating this valve the compressor may be prevented from 
sticking on a dead center, notwithstanding considerable variations 
in receiver pressure. 

A number of arrangements have been devised in the past to 
equalize the power and resistance, by varying with respect to one 
another the positions of the air and steam cylinders and their 
cranks. For example, in the earlier forms of the Burleigh, De la 
Vergne, and Ingersoll compressors, the cylinders, instead of being 
parallel to each other, were placed at 90 , with the cranks at 30 . 
In the old Rand and Waring, of 1876, the cylinders were set at 45 , 
the steam cylinder being of the oscillating pattern. The object of 
these and other similar devices was so to time the movements of 
the air and steam pistons that the power developed in the steam 
cylinder should be at its maximum when the air piston was just 
completing its stroke. But such constructions are deficient in 
strength and rigidity. They require heavier and more expensive 



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STRUCTURE AND OPERATION OF COMPRESSORS 3 1 

engine frames and foundations, and have not given satisfactory 
results. 

In the duplex type, as already explained, the lack of equali- 
zation between power and resistance is minimized, the most favor- 
able distribution and the highest degree of economy being attained 
in duplex stage compressors with compound steam cylinders. 

Proportions of Cylinders. It is customary to build compressors 
with a short stroke, as this is conducive to economy in compres- 
sion, as well as the attainment of a proper rotative speed. A short 
stroke is of special importance in simple straight-line compressors, 
because the power and resistance are more nearly equalized than 



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with a long stroke. The motion is less jerky and there is less 
liability of stopping on a center. With a long stroke, and relatively 
small diameter of cylinder, the piston would travel some distance 
under a constantly increasing resistance; then, after the discharge 
valves open, it would advance a considerable distance farther under 
a uniform resistance, while adding nothing to the amount of useful 
work. It should be noted, however, that the loss of capacity of the 
compressor due to piston clearance is less for a long than a short 
cylinder of the same diameter. In ordinary single-stage, slide- 
valve compressors the usual ratio of length of stroke to diameter 
of steam cylinder is 1-} to 1 or ij to 1. In some makes, such as the 
older Rand compressors, the ratio was considerably greater, varying 
from 1 J or if to 1. The length and diameter of steam cylinders in 
some recent designs are nearly equal. Quite different practice 



32 COMPRESSED AIR PLANT FOR MINES 

prevails, however, in the design of duplex Corliss compressors. In 
these are found such variations in the proportions of steam cylin- 
ders as: 12" X 30", 14" X 42", 20" X 42", and 30" X 60". 

The relative diameters of the air and steam cylinders depend 
obviously on the steam pressure carried and the air pressure to be 
produced. In mining operations there is usually but little varia- 
tion in these conditions. For rock-drill work, the air pressure is 
generally from sixty to eighty pounds. Of late, however, the appli- 
cations of compressed air for manufacturing purposes have so 
multiplied that some builders furnish compressors with steam and 
air cylinders of a great variety of proportions, for producing 
pressures of from ten to 120 pounds per square inch. 

Compressors Driven by Water-Power. When available, water- 
power furnishes a cheap and convenient means of driving air 
compressors. Impulse or tangential wheels, such as the Pelton, 
Knight, orRisdon,are best adapted for this service, the wheel being 
mounted directly on the crank-shaft, as shown by Fig. 21. This 
cut is of a 16" X 30" compressor, built by the Risdon Iron Works 
for the Goleta Mining Co. It is driven by a sixteen-foot wheel under 
a head of 300 ft. Figs. 22 and 23 show plan and elevation of an- 
other compressor by the same makers. Plants similar to this are 
built by the Compressed Air Machinery Co., Ingersoll-Rand Co., 
and other makers. Since the power developed is uniform through- 
out the revolution of the wheel, water-driven compressors should be 
of the duplex type, in order to equalize the resistance as far as 
possible. The rim of the wheel is made extra heavy, to supply the 
place of a fly-wheel. This is illustrated by Fig. 24, of an Inger- 
soll-Rand compressor driven by a Pelton wheel. 

To obtain the best efficiency, the peripheral velocity of an im- 
pulse wheel should be theoretically one-half the velocity of the jet 
of water from the nozzle. It follows that high heads of water in- 
volve correspondingly high peripheral velocities, and if the wheel 
be of small diameter a belt-drive would be required. But belting 
or gearing can generally be avoided, except when for any reason 
a turbine-wheel is adopted. Belt transmission is always disadvan- 
tageous, on account of the loss of power (say, eight to ten per cent.) 




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and the cost of deterioration of the belting. Practically in all cases 
an impulse wheel can be made of large enough diameter to run at 
a peripheral speed which will insure economical use of the water, 
while still giving a sufficiently low rotative speed for direct-con- 
nected compressor cylinders. In accomplishing this with very 
heavy heads, water-wheels are sometimes made of great size. 

Figs. 2? and 26 illustrate a well-known and interesting plant 
at the North Star Mine, Grass Valley. Cal., where a thirty-foot 
Pelton wheel drives a 300-horse-power, four-cylinder, two-stage 
compressor. The wheel makes sixty-five revolutions per minute 
under a head of 775 ft., with a single ij inch nozzle. The cyl- 
inders are single-acting (to obtain more efficient cooling of the air 
and make it easier to detect piston leakage) and measure 30 and 18} 
inches X 30 inch stroke. To avoid building excessively high 
foundations, as would otherwise be necessary for a wheel of this 
size, and also to furnish a substantial support for the gearing, the 
cylinders are set on angular frames at 3c 2 to the horizontal. In the 
lower left-hand corner of Fig. 25, the intercooler is shown, sub- 
merged in the tail-race, a feature of the plant of no small advantage 
in producing a thorough cooling of the air without expense. The 
spur-wheel on the main shaft, with its accompanying pinion, is 
provided for operating the compressor, if necessary at any time, by 
a synchronous electric motor (shown in outline to the left, in Fig. 
26). At the extreme left of Fig. 26 is a small auxiliary water- 
wheel, for starting the motor and putting it in synchronism.* 

At the Morning Mine, near Mullan, Idaho, is another large 
water-driven two-stage compressor, of an entirely different de- 
sign. There are four cylinders, a high- and a low-pressure being 
set tandem on each side of a set of three Pelton wheels, mounted 
on the crank-shaft. A large volume of water, under a head of 140 
feet, is delivered through S.ccc feet of flume and 400 feet of pressure 
pipe, driving two 12-foot wheels. Two other streams, piped respec- 

* The bevel gears driven from the spur and pinion do not form part of this 
plant. They were used at one time to drive another compressor. For a more 
detailed description of this compressor, the illustrations of which were kindly sent 
to the writer by its designer, Mr. E. A. Rix. see American Machinist. November 
10th, 1S9S, p. 831. 




Ph 






O 

u 

C 



bo 



o 

I— I 



3« 



COMPRESSED AIR PLANT FOR MINES 



tively ij and i mile, produce heads of 1,14c and 1,420 feet. These 
drive a 33-foot Pelton wheel (probably the largest in the world), 
placed on the compressor crank-shaft between the smaller wheels. 
The central wheel is driven by separate jets from the high-pressure 
lines, and on account of their difference in head, the diameter 
adopted for this wheel is a mean between the diameters theoretically 
necessary for obtaining a peripheral velocity properly proportioned 




Fig. 25. — Water-Driven Compressor at the North Star Gold Mine, California. 

(Side Elevation.) 



to each head. An actual mean peripheral speed of 8,000 feet per 
minute is attained. To control the water under these great heads, 
which correspond to pressures of about 490 and 610 pounds per 
square inch, slow-acting gate valves are provided, with by-passes 
for use in starting and stopping. The nozzles are arranged to be 
deflected clear of the wheel, in case it should be necessary to stop 
the compressor quickly. 

Each pair of cylinders are 33 \ and 18 inches respectively X 42- 



STRUCTURE AND OPERATION OF COMPRESSORS 



39 



inch stroke working at a piston speed of 560 feet. The low- 
pressure cylinders compress to about 30 pounds, the high-pressure 
to 90 pounds. Inter- and after-coolers are placed in the tail-race 
of the smaller wheels. A positive valve-motion is employed for 
both inlet and discharge valves, which are of the Corliss type. On 
each side, parallel to the center line of the compressor and geared 
to the crank-shaft, is a long shaft. Geared to the latter in turn are 




Fig. 26. — Water-Driven Compressor at the North Star Gold Mine, California. 

(Front Elevation.) 



short shafts which carry the valve eccentrics. As the discharge 
valves must open when the pistons are moving at nearly their 
maximum velocity (800 feet per minute), an auxiliary dash-pot is 
provided for allowing them to open automatically under the 
cylinder pressure, the positive eccentric motion closing them. 

Indicator cards from this compressor show it to be highly effi- 
cient. An average of a number of cards gives mean pressures of : 
low-pressure cylinder, 17.86 pounds; high-pressure, 41.14 pounds; 



40 COMPRESSED AER PLANT FOR MINES 

combined, 30.46 pounds. The mean theoretical adiabatic and 
isothermal pressures, corresponding to the combined mean are, 
respectively, 36.94 and 28.5 pounds. During the tests the ob- 
served temperatures were: cooling water, 38 ; air at discharge 
from low-pressure cylinder, 135 ; air at high-pressure inlet, 46 ; 
high-pressure discharge, 140 ; on leaving the after-cooler, 62 . 
Mean atmospheric temperature, 55 and of the cooling water 3 8°.* 

Provided there is a sufficient volume of water, impulse wheels 
may be used with quite low heads, by introducing multiple nozzles, 
directed tangentially at two or more points of the periphery of the 
wheel. To prevent the water from splashing over the compressor, 
the wheel is enclosed in a tight wooden or iron casing. The force 
of the water may be regulated by an ordinary gate-valve; but if 
the head be great it is always desirable to use a special slow-moving 
gate fas noted above), to avoid danger of rupturing the pressure 
pipe in case the compressor is suddenly stopped. Turbines are 
obviously not so well adapted for operating compressors as the 
impulse wheels. A method of compressing air by the direct action 
of falling water is described in Chapter XV. 

Belt-Driven and Geared Compressors. These are often con- 
venient, and are furnished in a number of styles and sizes by com- 
pressor-builders. The fly-wheel is replaced by a large belt- wheel, 
with an extra heavy rim to give it sufficient weight, Fig. 27. Power 
may be derived from an engine already installed for other purposes, 
or from a water-wheel or electric motor. Since electric transmission 
of power has come into general use in mining regions, a belt-drive 
from a motor is frequently advantageous when there is sufficient 
floor space. Some of the compressor-builders have introduced a 
"silent-chain'' drive, for use when it is desired to place the motor 
close to the compressor and on the same bed-frame, and at the 
same time avoid the use of gearing. It has a high efficiency (about 
ninety-five per cent.) and may be employed for transmitting up 
to, say, 200 horse-power. 

Although a belt-drive is preferable to gearing, at least for a com- 

*This plant, described in American Machinist, September 26th, 1901, was, 
like that at the North Star mine, designed by Mr. E. A. Rix. 



STRUCTURE AND OPERATION OF COMPRESSORS 



41 



pressor erected on the surface, geared electric-driven sets are some- 
times used, a spur-gear on the crank-shaft engaging with a pinion 
on the armature. Single-reduction gearing will generally answer. 
This design has been adopted even for large plants, as, for example, 
at a recent two-stage installation of the Compafiia de Pefioles, 
Mexico. By giving sufficient diameter and weight to the spur- 




Fig. 27. — Ingersoll-Sergeant Straight-Line, Belt-Driven Compressor. 



wheel, it not only produces the low piston speed necessary, but 
serves also as a fly-wheel. Raw-hide pinions are desirable to 
reduce the noise of the gearing. Induction motors are suitable for 
such service, as they are capable of running economically under 
wide variations of load. It may be added that the small, high- 
speed Christensen compressor is well adapted for gearing directly 
to a motor, thus forming a very compact machine for uses where 
lightness or portability is essential. Under proper conditions, an 
electric -driven compressor may be erected underground, near the 
point of application of the air power. Though some loss is in- 
evitable in converting electric energy into compressed-air power, 



42 



COMPRESSED AIR PLANT FOR MINES 



this may be offset in some circumstances by considerations of 
convenience of installation. When the electricity is generated by 
water-power in large quantities, as in many Western mining dis- 
tricts, the cost per horse-power compares favorably with that of 
steam-driven compressors. 

Note. It is unnecessary, and in fact hardly practicable, in a 
book that is not intended to be a trade publication, to describe 
separately and in detail all the numerous makes of air compressors. 
In the foregoing chapter some of the well-known compressors are 
instanced, for the purpose of illustrating various features of design. 
The same remark may be made with regard to the descriptions of air 
valves, etc., in Chapters VII, VIII, and IX. It must not be under- 
stood, however, that the compressors specifically referred to in this 
book are considered the only good ones, nor that the author, by 
omitting to mention and to insert cuts of all compressors, desires 
thereby to discriminate against those that^ are perhaps less well 
known only because they are the product of recently established 
builders. Most of the compressors made in Europe, including 
many excellent machines, are omitted altogether, though references 
to interesting features of the valve-motions of some of them will 
be found under the appropriate heads. 

An alphabetical list, which, while incomplete, comprises the 
names of most of the American compressor-builders, is given below. 



Allis-Chalmers Co. 

American Air Compressor Works. 

Chicago Pneumatic Tool Co. (Frank- 
lin compressor). 

Christensen Motor-Driven Compressor 
(Allis-Chalmers Co.). 

Clayton Air Compressor Works. 

Compressed Air Machinery Co. 

Franklin Iron Works. 

Heron & Bury Manufacturing Co. 



Ingersoll-Rand Co. 

Knowles Steam Pump Works. 

Laidlaw-Dunn-Gordon Co. 

Leyner, J. Geo., Manufacturing Co. 

McJviernan Drill Co. 

New York Air Compressor Co. 

Nordberg Manufacturing Co. 

Norwalk Iron Works Co. 

Rix Compressor and Drill Co. 

Sullivan Machinery Co. 



Vulcan Iron Works. 



CHAPTER III 

THE COMPRESSION OF AIR 

In the production and use of compressed air occur serious losses, 
which to a large extent are unavoidable. Even in the best com- 
pressors the efficiency, or ratio of the force stored up in the com- 
pressed air to the work which has been expended in compressing 
it, rarely exceeds seventy- five per cent, and often falls below sixty 
per cent. To understand the causes of these losses it will be neces- 
sary to study the principles involved in the operation of compres- 
sing air. This study is advisable, also, before proceeding to a 
description of the air end of the compressor. Several definitions 
may first be given : 

"Free air" is a term commonly used in dealing with the subject 
of air compression. It is simply air at normal atmospheric press- 
ure, as taken into the cylinder of the compressor. But since 
atmospheric pressure varies with the altitude above sea-level, and 
with the barometric reading at any particular time or place, it 
follows that the expression "free air" has no precise general sig- 
nification, with respect to the pressure, volume, and temperature of 
the air. At sea-level it is in reality "compressed air," at the normal 
atmospheric pressure of 14.7 pounds per square inch. As com- 
monly employed the term means air at sea-level pressure, and at 
a temperature of 6o° Fah. 

The absolute pressure of air is measured from zero, and is equal 
to the assumed (or observed) atmospheric pressure plus gauge 
pressure; ordinary gauges registering pressures in pounds per 
square inch above atmospheric pressure. 

Absolute temperature is the temperature as measured from the 
"absolute zero" point, which is 491.4 F. below the freezing-point 
of water, or say 459 below zero Fahrenheit. For example, 6o° F, 

43 



44 



COMPRESSED AIR PLANT FOR MINES 



of thermometric temperature is equivalent to an absolute tem- 
perature of 459° + 6o°=5i9° F. 

There are two fundamental laws governing the behavior of air 
and gases under compression, and according to which the relations 
existing between volume, pressure, and temperature may be ex- 
pressed. The first law (Boyle's) is: At a constant temperature the 
volume occupied by a given weight of air varies inversely as the 
pressure. This condition is expressed by the equation : 

P' V 
P V—P' V — constant; or p — v , ; in which 

F = the volume of the given weight of air (or gas) at the freezing- 
point and at a pressure P; V = the volume of the same weight 
of air at the same temperature and at any pressure, P' (the pres- 
sures being absolute pressures). 

For example, if a quantity of atmospheric air be compressed 
at constant temperature to 0.147 °f its original volume (the atmos- 
pheric pressure being 14.7 pounds), a pressure of 100 pounds per 
square inch is obtained; when compressed to 0.074 of its original 
volume, the pressure becomes 200 pounds, and so on. 

The following table* shows the weight and volume of dry air, 
at temperatures from o° to 212 F., and at atmospheric pressure: 

Table I 



Temperature 
Degrees f ah. 


Weight of 

one Cubic Foot 

in Pounds. 


Volume of one 

Pound in 

Cubic Feet. 


Temperature, 
Degrees Fah. 


Weight of 

one Cubic Foot 

in Pounds. 


Volume of one 

Pound in 

Cubic Feet. 





.0863 


11.582 


no 


.0697 


14-345 


10 


.0845 


11.834 


120 


.0685 


14.596 


20 


.0827 


12.085 


130 


.0674 


14.847 


3° 


.0811 


12.336 


140 


.0662 


15.098 


3 2 


.0807 


12.386 


150 


.0651 


!5-35° 


40 


.0794 


12.587 


160 


.0641 


15.601 


5° 


.0779 


12.838 


170 


.0631 


15-852 


60 


.0764 


13.089 


180 


.0621 


16.103 


62 


.0761 


13. 141 


190 


.0612 


i6-354 


7° 


.0750 


I3-340 


200 


.0602 


16.605 


80 


.0736 


L3-59 2 


210 


-°593 


16.856 


90 


.0722 


13-843 


212 


.0591 


16.907 


100 


.0710 


14.094 









* From D. K. Clark and Appleton's " Applied Mechanics." 



THE COMPRESSION OF AIR 45 

The production and use of compressed air, if governed solely 
by the law stated above, would be a simple matter. But during 
compression heat is generated, and when the air is allowed to ex- 
pand back to its original volume this heat is given up. The 
internal work, manifested by the development of heat, is independ- 
ent of the time occupied by the compression. This condition is 
expressed by the second law, that of Charles and Gay-Lussac,"Z/fz.: 
When under constant pressure, the volume of a gas expands or 
contracts for each degree rise or fall of temperature, from freezing 
to boiling, by a constant fraction of the volume which it occupied at 
the freezing-point. Expressed in another way, the volume of a 
gas under constant pressure is nearly proportional to the absolute 
temperature. The equation may be written: V' = V (i X at°). 
The complete relations between pressure, volume and tempera- 
ture are expressed by the equation: P'V' = PV (i X at°), in which 
P' and V represent the pressure and volume of a given weight of air 
(or gas) at t° Fah. above the freezing-point, P and V the pressure and 
volume of the same quantity of air at the freezing-point, and a the 
coefficient of expansion of air, which is practically constant and 
is very nearly ^-^ on the Fahrenheit scale. Hence, for a rise in 
temperature of i° F., the volume of the air increases by 4^3- of the 
volume occupied at the freezing-point, under the same pressure, 
49 1 ° F. being the absolute temperature below freezing. 

The practical application of this law is that the development 
of heat reacts upon the air under compression, and increases the 
pressure which would be due merely to the reduction in volume. 
By cooling the compressed air to its original temperature the press- 
ure would be reduced to the normal amount, according to the first 
law. That is, the heat produced by the compression of a given 
volume of air corresponds in degree to the cold resulting from 
the expansion of the same quantity of air back to its original 
volume and pressure. It is evident that this property of air 
has an important application in the production and use of com- 
pressed air. 

Two other statements may be deduced from what precedes: 
1. Under constant pressure the volume of air varies directly as the 



46 



COMPRESSED AIR PLANT FOR MIXES 



absolute temperature; 2. The volume being constant, the absolute 
pressure varies directly as the absolute temperature. 

The heat generated during compression and corresponding to 
different pressures is shown by the following table, the volume at 
normal atmospheric pressure being 1, at a temperature of 6o° Fah. : 



Table II 



Pressure in 
Atmospheres. 


Absolute Pressures. 

Pounds per Square 

Inch above 

Vacuum. 


Volumes in 

Cubic Feet, 

Adiabatic 

Compression 


Final 
Temperatures, 
Degrees Fah. 


Corresponding 

Increases of 

Temperature. 


1. 00 


14.70 


1 .000 


60.0 


00.0 


I.25 


18-37 


O.854 


94-8 


34-8 


1-50 


22.05 


0.750 


124.9 


64-O 


2.00 


29.40 


0.612 


175-8 


115. 8 


2.50 


36.70 


0.522 


218.3 


15S.3 


3.00 


44.10 


0.459 


255.1 


ig>.i 


3-5° 


51.40 


0.411 


287.8 


227.8 


4.00 


58.80 


0-374 


3*7-4 


2 57-4 


5.00 


73-50 


0.319 


3°9-4 


309-4 


6.00 


88.20 


0.281 


414-5 


354-5 


7.00 


102.90 


O.252 


454-5 


394-5 


8.00 


117.60 


0.229 


490.6 


430.6 


9.00 


*3 2 -3° 


0.211 


5 2 3-7 


463 . 4 


10.00 


147.00 


0-I95 


554-o 


494-o 


15 . 00 


220.50 


0-147 


681.0 


621.0 



From this table it is seen that the rate of increase of temperature 
is not uniform, but diminishes as the pressure rises. Thus, from 
1 to 2 atmospheres the increase is 115. 8°; from 2 to 3, 79.3 ; from 
3 to 4 atmospheres, 62. 3 , etc. The quantity of heat developed 
during compression may be calculated by the following formula:* 

RX/ XT . V . , . . 
Q= — = — X Nap. log.— , in which 

Q = quantity of heat in thermal units (calories). 

R = constant = 96.03 7 (French unit) or 52.96 (English unit). 

t= absolute final temperature in degrees centigrade, corre- 
sponding to V'. 

J = value of one thermal unit— 1,390 foot-pounds (or 772 foot- 
pounds if English units be used). 



* Zahner, " Transmission of Power by Compressed Air," p. 109. 



THE COMPRESSION OF AIR 



47 



V and V = volumes of air in cubic feet, at beginning and end 
of compression. 

As the rise in temperature due to compression is proportional to 
the ratio of the final absolute pressure to the initial absolute pressure, 
the quantity of heat generated during compression at high altitudes 
to any given pressure, and _ volumes 

the consequent loss of 
work, is greater than at 
sea -level. 

The diagram, Fig. 28, 
is taken from ''Compressed 
Air Production," by W. L. 
Saunders. It is, in real- 
ity, two diagrams, com- 
bined to save space. First, 
beginning at the lower left- 
hand corner, and curving 
upward, are the adiabatic 
and isothermal compression 
lines. Their intersections 
with the horizontal and ver- 
tical lines give the volumes 
of the unit of air when sub- 
jected to any given pressure, 
by reading the figures at 
the top, and right- or left- 
hand margin of the dia- 
gram. The initial volume 
is taken as 1 , and the spaces 
between the horizontal and vertical lines are each one-tenth. 
The resulting volume is independent of the initial tempera- 
ture of the air. The corresponding pressure may be read 
in terms of either gauge or atmospheric pressure. Second, be- 
ginning at the lower right-hand corner of the diagram, and rising 
toward the left, are the lines of temperature, the assumed initial 
temperatures being o°, 6o° and ioo° F. The temperature corre- 



2L 
20 
19 
18 
17 
10 
15 
14 

CO 



S3 13 

-a 

p. 

S 12 

a 

3 11 
>» 

x> 

gio 

2 9 

h 

8 




\ 




\ 
















294.0 
279 3 




\ 


\ 


















264.5 




\ 


\ 


















249.9 




\ 


\ 


















235 2 




\ 


\ 




\ 














220 5 










\ 












205.8 










\ 












191.1 










\ 












170.4 




















1 


161.7 






c 


3>\ € 


\ 


p\ 








1 


147.0 










A 






Oil 




132.3 








<s>\ 


<s>\ 

. 1\ 


A 




C 


/ 




117.6 








O 


*\ a 


\ c 


j\ 


J/ 


=0/ 

84- 




102.9 












\ ' 


i\ 


*7 

7 


5/ 




88.2 
73.5 
58.8 
44.1 
29.1 
14.7 
0.0 


6 
5 
4 


















*7 












































3 










•v> 


4 


3&X 


v 


\ 




2 
1 






< 


o\*2 








\\ 


A 




















\ 



Temperature Fahrenheit 

Fig. 28. 



48 COMPRESSED AIR PLANT FOR MIXES 

sponding to any given pressure is read on the lower margin. It 
should be observed that these heat curves are those of adiabatic 
compression. 

It follows from the results obtained above that if the temperature 
of the air be allowed to rise during compression a loss of work ensues. 

Modes of Conducting the Operation of Compression. Theo- 
retically, air may be compressed in two ways : 

1. The temperature may be kept constant during compression, 
the heat generated being abstracted by cooling devices as fast as it 
is developed. In this case the pressure of the air varies according 

P' V 

to the equation P V = P'\ , or — = — , and compression takes 

place isothermally ; that is, the compression curve of an indicator 
diagram would be an isothermal curve. 

2. The temperature may be allowed to rise unchecked during the 
period of compression; there is no transference of heat, either by 
radiation or by cooling devices. The rise in temperature increases 
the pressure that would be due to reduction of volume only. In other 
words, the pressure rises faster than the volume diminishes, and 

P' V 

— is no longer equal to, but is greater than — . To form an equa- 
tion, — must be increased. This is done by introducing an ex- 

V 

ponent w, which raises all values of — to a power whose index has 

p' /V \ n = I -4°^ 

been found to be 1.406. This gives — = ( — ) , which is 

the equation of adiabatic compression. (The specific heat of air 
at constant pressure is 0.2375, and its specific heat at constant 
volume is 0.1689. The exponent n is the ratio between these 

specific heats, viz.: \ ; J = 1.406.) 

.1689 

The relations between the two conditions of compression is 
shown graphically by Fig. 29. By laying off to scale the volumes 
of air on the horizontal line of the diagram, the corresponding 
pressures at different points of the stroke of the compressing 
piston are measured on the verticals. The adiabatic curve rises 



THE COMPRESSION OF AIR 



49 




Fig. 29. 



more rapidly than the isothermal, according to the law. There- 
fore, in compressing adiabatically a quantity of air to a given 
volume, more work is expended than if the compression were 
effected isothermally. Perfect isothermal compression cannot be 
attained in practice. Even with the best cooling arrangements the 
compressor would have to run at an extremely slow speed, and be 
of very large size, to approach closely the condition of isothermal 
compression. On the other hand, if the air 
compressed adiabatically could be kept hot 
until used, the loss of the additional work 
which was expended in compressing it 
would be prevented. But neither can this 
be done. The air 
is almost always 
conveyed to con- 
siderable distances — 
before it is used, 
and the loss of heat by radiation from the pipes soon reduces the 
pressure to that corresponding with the temperature of the sur- 
rounding atmosphere. In practice, therefore, neither of these 
theoretical methods of compression is possible; a combination or 
modification of the two is employed, the net result depending upon 
the degree of perfection of the compressing engine and of the cool- 
ing arrangements provided. 

As shown on the diagram, the actual line of compression must 
lie somewhere between the adiabatic and isothermal lines. When 
compressing in a single cylinder to sixty or eighty pounds pressure, 
and at a piston speed not exceeding 300 feet per minute, it is 
probable that about one-half of the total possible cooling is all that 
may be expected.* The aim is to begin compression with the air 
at a low initial temperature, and to bring the compression line as 
close as possible to the isothermal line. Next, it is of the utmost 
importance that the air shall be cooled thoroughly during com- 
pression and before it leaves the cylinder. Any subsequent cooling, 
whether in the receiver or in the air main, must entail loss. 

* Frank Richards, " Compressed Air," p. 66. 
4 



50 COMPRESSED AIR PLANT TOR MIXES 

As a matter of fact, the abstraction of heat during compression 
in ordinan- practice is very imperfect. Some distance must be trav- 
ersed by the piston, in compressing the air, before there is any 
considerable rise in temperature, and until the temperature does 
rise no cooling can be effected. In other words, the abstraction of 
heat does not begin at the beginning of the stroke. The temper- 
atures of the air taken into the cylinder and of the water used for 
cooling are likely to be nearly the same, so that all the possible 
reduction of temperature in any one cylinderful of air must take 
place in a period of time less than that occupied in making the 
stroke. Most of the cooling is done necessarilv in the latter hah 
of :he stroke. It should be noted, moreover, that soon after the 
compressor begins running the cylinder itself becomes quite hot 
and heats the air during intake. For this reason the total amount 
of cooling to be effected is greater than that which is required to 
abstract the heat developed during the compression of a given 
volume of air to a given tension. In modern dry compressors of 
fairly large size, and running at full working speed, the com- 
pression line is usually much nearer the adiabatic than the iso- 
thermal curve, and often follows the adiabatic curve quite closely. 

There are two methods of absorbing the heat produced by 
compression : 

i . Bv introducing: cold water into the air cvlinder. 

2. By cooling the cylinder from without, enveloping it in a 
cold-water jacket. 

Machines of the first class are known as "wet compressors"; 
those of the second, "dry compressors." 

The values of the coefficient n in the equation already given, 
P' fX 



, > have been found for the dirterent svstems of compres- 

sion. As has been stated, in the case of purely adiabatic compres- 
sion, with no cooling arrangements, n = 1.406: in ordinan* single- 
cylinder dry compressors, provided with a water-jacket, n is 
roughly 1.3, while in the best single-stage wet compressors (with 
spray injection) n becomes 1.2 to 1.25. In the poorest forms of 
compressor the value ?z = i.4 is closely approached. It should be 



THE COMPRESSION OF AIR 



51 



added that for large well-designed compressors with compound air 
cylinders and efficient intercooling, the exponent n, referred to the 
combined indicator cards, may be as small as 1.15. This result 
has been obtained, for example, from a 2,00c horse-power, two- 
stage compressor at Quai de la Gare, Paris. 



Compression 
Without Cooling 




Fig. 30. 



Effect of 
Water Jacket 




Fig. 31. 



Effect of 
Spray Injection 




Fig. 32. 

The diagrams, Figs. 30, 31, and 32, show the relative positions 
of the several compression lines, the areas between the compression 
and isothermal lines being shaded in each case. These are not 
actual indicator diagrams. They are intended approximately to 
represent the relations between the different lines, under the con- 
ditions named. 



CHAPTER IV 

WET COMPRESSORS 

Although during the past fifteen years wet compressors have 
become almost obsolete in the United States, it is necessary to give 
some attention to them, not only because many are still used in 
Europe, but also because a discussion of their design and operation 
will lead to a better understanding of the comparative merits of 
the systems of cooling employed in the modern dry compressors. 

Wet compressors are of two kinds . 

i. The so-called hydraulic- plunger compressors, in which water 
is introduced in bulk into the air cylinder, and is injected also in 
the form of spray, 

2. Those in which the cooling water is injected in the form 
of fine spray or jets only. 

Compressors of the first type comprise some of the earliest forms 
of air compressor. One of the best of this class is the modernized 
Dubois-Francois, built at Seraing, Belgium. It has been widely 
used in Europe, for mining and tunnelling operations, and it is 
worth noting that, up to about 1877, one of them was employed at 
the Sutro tunnel, Nevada. Another well-known compressor of 
the same class, but of different design, is the Humboldt, made at 
Kalk, near Cologne, Germany. One of these also was erected 
at the Sutro tunnel, and did excellent work. A brief description 
of the old Humboldt compressor (Fig. 33) will serve to explain the 
principle and construction of these machines. 

The water constitutes a piston for compressing the air; an 
ordinary plunger, like that of a pump, moving in a horizontal 
cylinder filled with water. At each end of the cylinder, and con- 
nected with it by an easy curve, is a vertical air chamber. The 
upper ends of these chambers are provided with the necessary air 

52 



WET COMPRESSORS 



53 



inlet and discharge valves. As the piston reciprocates, the air 
is drawn alternately into one air chamber and compressed in the 
other, bv the rise and fall of the water level. At the end of each 
stroke the air compressed by the rising mass of water in the air 
chamber passes through the discharge valves into the receiver. 
As the air is in contact with the water a partial cooling is effected, 
and to prevent the water itself from becoming heated a constant 
circulation must be maintained. A further cooling is brought 
about by the injection of sprays from a small force pump into the 
cylinder and vertical air chambers. The pump is operated from 




Fig. 33. — Humboldt Wet Compressor. 

the cross-head of the compressor itself. This type of compressor 
is simple, and if the sprays be copious the air is quite effectually 
cooled; but it is generally limited to rather slow speeds (only ioo 
to 150 feet piston speed per minute or less in some cases), on ac- 
count of the inertia of the body of water. This is about one-third 
to two-fifths of the piston speed of modern dry compressors, and 
it follows that such engines are comparatively heavy and bulky for 
a given output of air, besides requiring expensive foundations. It 
is claimed, however, that a more recent form of Humboldt wet 
compressor can be run successfully at speeds of 300 to 360 feet per 
minute, the temperature of the air at discharge being kept at 77 to 
8o° Fah.* This is such remarkably good work that the results are 
open to question, as far as regular, normal service is concerned. 
Lower speeds are certainly advisable for this form of compressor. 

* P. R. Bjorling, Colliery Guardian, Oct. 2d, 1896, pp. 629-630. 



54 



COMPRESSED AIR PLANT FOR MINES 



The machines are made of large size and are heavily and sub- 
stantially built. Violent shocks are apt to be caused by attempting 
to run at high speeds, for which reason the vertical air chambers 
join the cylinder with a curve of long radius to ease the movements 
of the mass of water. 

Fig. 34 shows a late type of the Hanarte wet compressor, many 
of which have been built for French and Belgian mines, and also 
for use in connection with ice-making plants. They are generally 
of large size, and are found to be highly efficient when run at piston 




IG. 34. — Hanarte Wet Compressor. 

speeds of 250 to 275 feet per minute. An advantage of the splayed 
out vertical ends of the cylinders is that large inlet and delivery 
valves can be placed in the cylinder cover or head, moving vertically 
and being readily accessible. Sprays are used in addition to the 
water in bulk. 

A difficulty with wet compressors of this class is that an efficient 
circulation of cold water is not easy to maintain. Only a small 
quantity of fresh water can be injected at each stroke, and without 
copious sprays the cooling is imperfect. This is due to the fact 
that, although the mass of water kept in motion in the cylinder and 
air chambers is large, there is between it and the air only a surface 
contact. Since water is a poor conductor of heat, under these 
conditions it can hardly be questioned that the air is cooled more 
by contact with the relatively large area of the wet cylinder walls 
than by its contact with the small superficial area of the rising and 



WET COMPRESSORS 55 

falling water. Another disadvantage is that the compressed air 
delivered from the cylinder is practically saturated with moisture. 

Compressors of the second class, in which the cooling water is 
used only in the form of jets or spray, constitute an improvement 
upon the older design, in being much less cumbrous and per- 
mitting a higher working speed. This method of cooling was first 
applied by Colladon, at the St. Gothard tunnel. Though these 
compressors are still frequently used in Europe, they have given 
way in great measure to dry compressors, and in American practice 
have become almost obsolete. The air cylinder does not differ 
materially from that of the dry compressor. A small water pipe is 
tapped into each cylinder head and fine spray is injected in front 
of the piston while compression is taking place. 

Undoubtedly this system is superior to that involving the use of 
water in bulk. Since the water is in a state of fine division a rela- 
tively large surface of contact is presented, and the air is kept 
thoroughly saturated with moisture during compression. Zahner, 
in his " Transmission of Power by Compressed Air," p. 28, states 
that Colladon's St. Gothard compressors, " which were run at a 
piston speed of 345 feet, and compressed the air to an absolute 
tension of 8 atmospheres (103 pounds gauge pressure), gave an 
efficiency which never descended below 80 per cent, while the 
temperature of the air never rose higher than from 12 to 15 C. 
(53° to 59° F.)." The temperature of the injection water is not 
stated, but must have been very low to obtain such remarkable 
results. 

A dry compressor may be converted into a wet compressor 
merely by providing the cylinder with water jets. The injected 
w r ater collects in the cylinder until enough is present to fill the piston 
clearance space at the end of the stroke. Then any additional 
amount of water is forced out at each stroke with the compressed 
air through the discharge valves into the air receiver. From the 
receiver the water is drained away from time to time. As the pis- 
ton clearance in well-designed compressors is extremely small, very 
little water can remain in the cylinder to be churned back and forth 
by the piston. The water used for injection should be as pure and 



56 



COMPRESSED AIR PLANT FOR MINES 



cold as possible. Gritty water must never be employed, as it 

would injure the cylinder, piston and valves. 

A proper injection apparatus should fulfil three conditions: 
i. The injection must commence at the beginning of the 

stroke and continue to the end, against the advancing piston. 

2. There should be a thorough diffusion of the water in the 
form of spray throughout the cylinder. By mere surface contact 
water takes up but little heat. Even a single strong jet is quite ef- 
fectual, however, because on striking the piston it is thoroughly 
broken into spray. 4» 

3. A definite volume of water should be injected, the quantity in- 
creasing with the pressure under which the compressor is working; 
that is, with the quantity of heat generated. If the quantity of 
water used be insufficient to abstract the heat, a large amount of 
moisture is taken up by the warm air and carried into the receiver 
and piping. 

The heat units developed by compression having been cal- 
culated, the quantities of water required for different pressures 
are shown in the following table.* The average temperature of 
injection water may be taken as, say, 68° Fah., and is considered 

Table III 



Pressures. 


Heat units 

developed by 

compression 

in one pound of 

free air. 


Pounds of water to be in- 
jected at 68° F. to keep final 
temperature at 104 F. 




Gauge 
pressure . 
Pounds. 




Above 

vacuum. 

Atmospheres. 


Per pound of 
free air. 


Per cubic foot of 
free air. 


2 

3 
4 

5 
6 

7 
8 

9 
10 
12 


14.7 
29-4 
44-i 
58.8 

73-5 

88.2 

102.9 

117. 6 

1 3 2 -3 

161. 7 


58. 3 10 

92.390 

116.627 

135-388 

151.700 

163.735 
174-937 
184.865 
193.701 
209.090 


0-734 
1. 164 
1.469 
1. 701 
1. 891 
2.063 
2.204 
2.329 
2.440 
2.634 


0.056 
0.089 
0.112 
0.130 
0.144 
0.158 
0.168 
0.178 
0.186 
0.201 



* This table is taken in part from that given by Zahner, "Transmission of 
Power by Compressed Air," p. no, English units being substituted for French. 



WET COMPRESSORS 57 

as having accomplished its work if it leaves the cylinder at 104 
Fah., these temperatures corresponding, respectively, to 20 and 

4 G°C. 

There is no practical advantage to be gained by using an 
excessive quantity of water, and care should be taken to inject no 
more than is required. The additional cooling effect of a greater 
mass of water in the cylinder would be but small — as has been re- 
marked under wet compressors of the first type — and more power 
would be consumed in pumping the water into the cylinder and 
then forcing it out again through the delivery valves. 



CHAPTER V 
DRY COMPRESSORS 

In the dry system of compression no water enters the air cylin- 
der except that which is carried as moisture in the air itself. All 
the cooling during compression, aside from radiation, is effected by 
a water envelope, or "jacket," surrounding the cylinder, and in 
which cold water is kept constantly circulating. 

Fig. 35 shows the longitudinal section of a Xordberg jacketed 
air cylinder. (Reference may also be made to Figs. 2, 5, 7, 10, 19, 
and other cuts of longitudinal sections, as illustrating different 
types of jacketed cylinders.) The cylinder is enclosed in an outer 
shell, leaving an annular space, J J, to be occupied by the water. 
Besides the annular jacket nearly one-half the area of each cylinder 
head is also covered by water jackets, K K. The remainder of the 
end areas is occupied by the suction and delivery valves, as shown. 
The air-deliven- valves are sometimes placed radially, close to the 
cylinder ends, whereby a larger proportion of the area of the heads 
can be jacketed. This is true, for example, of one or two of the 
La idlaw- Dunn -Gordon patterns. 

In Fig. 35 the circulation of water is effected by pipes connecting 
with the openings A and B, respectively for inlet and discharge. 
To cause a proper circulation the spaces enclosed by the jacket are 
subdivided. The cold water enters at A, and after circulating 
through the annular and end jackets J J, K K, is finally discharged 
at B. The smaller jackets on the cylinder heads are design; d to 
surround the valves and air passages as completely as possible, in 
order to exert the maximum degree of cooling. At C is a drain 
pipe through which the jacket is blown out occasionally to clear 
it of sediment. 

In some makes of compressor, the annular jacket is divided by 



DRY COMPRESSORS 



59 



vertical partitions, so that the cold water entering at the top 
passes first around about one-fifth of the length of the cylinder 
nearest each end. The water then circulates around the middle 
portion, and is discharged at the top. Although in this arrange- 
ment the fact is recognized that at the end of the stroke, where the 
air pressure is highest, the greatest amount of heat is generated; 
still, in some of the same designs little, if any, of the cylinder-head 




r 

Fig. 35. — Air Cylinder of Nordberg Compressor. 

area is jacketed, because of the mode of placing the inlet and dis- 
charge valves. This would seem to be a defect because, on 
approaching the end of the stroke, the piston rapidly covers the 
annular jacket, leaving a very small part of its area available for 
cooling the hot compressed air while being discharged from the 
cylinder. It is at this point of the stroke that large end jackets are 
most valuable. 

The jacket of one of the Laidlaw-Dunn-Gordon designs (Fig. 
36) is cast with eight longitudinal partitions, extending alternately 
from each end of the cylinder nearly to the opposite end. The 
water, which enters near the top, is forced to travel back and forth 
between the partitions and from one end of the cylinder to the other 



6o 



COMPRESSED AIR PLANT EOR MIXES 




DRY COMPRESSORS 6l 

until it is finally discharged. An active circulation is thus main- 
tained. For furnishing the cooling water a tank is often provided, 
set at some elevation above the compressor, or a small pump may 
be employed. 

Naturally, a partial cooling only can be effected by water-jacket- 
ing the air cylinder. Much depends on the speed at which the com- 
pressor is run. In the best single-stage compression, to say seventy 
or seventy-five pounds, and at not over 300 feet piston speed, it is 
doubtful whether more than about one-half of the total possible 

P' /V\* 
cooling can be effected ; that is, in the equation — = ( — J , n would 

be equal to, say, 1.22 to 1.25. Heat is generated faster than it can 
be abstracted, and only a portion of the volume of air passing 
through the cylinder comes into direct contact with the cooling sur- 
faces. It is important, therefore, that as much as possible of the 
total cylinder surface be covered by the jacket, and that the piston 
speed be moderate. But, in a dry compressor, as the air is com- 
paratively free from moisture, some heating is not so objectionable 
as it would be in a wet compressor. As a matter of fact, the cylin- 
der, discharge pipe, and even the receiver, are usually quite hot 
when the compressor is running at full speed; often too hot to be 
touched with the hand. In a plant at Birmingham, England, with 
well-jacketed cylinders, and compressing only to forty-five pounds, 
a temperature of the air at delivery has been observed as high as 
28c F. In this case the compressor is large, so that the super- 
ficial area of the jackets is small as compared with the volume of 
the cylinder. It is probable that the heat of compression in 
dry compressors ranges from 200° to a maximum of 400 F. for 
the ordinary pressures used in mining, though it does not often 
exceed 350 . Care should be taken not to allow the temperature 
to rise above this point.* At a large mine in Montana, the writer 
has observed the thin wrought-iron delivery pipe of a fifty-drill 
compressor red-hot for a distance of nearly six inches from the 
cylinder shell. Driving compressors at too high a speed (when not 

* T. G. Lees, Trans. Federated Inst. Mining Engs., Vol. XIV, p. 569. See 
also Chapter XIII of present volume. 



62 COMPRESSED AIR PLANT FOR MIXES 

large enough for their work) is often the cause of the poor results 
complained of by some users of compressed air. 

In some compressors the inner shell of the air cylinder, i.e., 
between the cylinder and water-jacket, has been made of hard 
brass, which by its high conductivity assists in carrying off the 
heat. With the same end in view, the cylinder walls should be as 
thin as is consistent with safety. 

Besides its function of cooling the air during compression, the 
water-jacket of a dry compressor is indispensable from a mechanical 
point of view, in keeping down the temperature of the cylinder 
shell. Without some special provision for cooling the cylinder 
the metal would become hot enough to burn the oil, and render 
proper lubrication impossible. To furnish a larger cooling sur- 
face one of the older styles of Rand compressor had a hollow 
back piston-rod and hollow piston, through which water is circu- 
lated. To maintain circulation the back piston-rod worked tele- 
scopically in a stationary tube connected with the water supply. 

Piston Clearance in the Air Cylinder. In even* engine, whether 
steam engine or compressor, the amount of clearance at the end of 
the stroke, between the piston and cylinder head, is a matter of 
some importance. It has a special bearing in the case of a dry 
compressor, which may be explained as follows. Toward the 
end of the stroke the compressed air in front of the piston begins 
to pass through the deliver}- valves as soon as its tension exceeds 
that of the air in the discharge pipe leading from the cylinder to 
the receiver. But remaining in the clearance space, on the com- 
pletion of the stroke, is a certain quantity of warm compressed 
air, which in the case of a dry compressor can never be discharged. 
On the back stroke the clearance air expands and partly fills the 
cylinder behind the piston. Xo air can enter through the inlet 
valves until the pressure inside the cylinder falls below atmos- 
pheric pressure. It is never possible, therefore, to take a full 
cvlinder of fresh air even under the best conditions, and the clear- 
ance space must be made as small as possible, say, about one- 
sixteenth inch. Or, the clearance may be expressed as a ratio, by 
dividing the clearance volume by the entire cylinder volume swept 



DRY COMPRESSORS 



63 



through by the piston in making its stroke. In a wet compressor 
the clearance space is filled with water, and therefore does not pro- 
duce the effect just described. 

Mr. W. L. Saunders states :* " The clearance space in modern 
air compressors of the best design (including counter-bore and dis- 
charge valve clearance) varies from .002 to .0094 of the volume of 
free air furnished by the cylinder. The variation is somewhat 
dependent upon the length of stroke. At seventy-five pounds 
pressure, and making due allowance for increased volume of air due 
to heat, the clearance loss of volume varies from .01 to .047, or 
from one to five per cent, of the air when compressed." 

It maybe added that, in compressors of some makes, the clear- 
ance reaches ij and even as high as 2 to 2 J per cent, of the 



Air Card showing 
effect ofLclearance. 
Volume Between b and c 




-)- Atmospheric 
5 [C Line 

'- — Vacuum 

Fig. 37. 

piston displacement. The lowest figures given above apply to 
large, long-stroke compressors; the higher to the small, short- 
stroke machines in common use for many kinds of service. 

The diagram, Fig. 37, shows the effect of clearance. Before the 
inlet valves can open, the piston must travel from c to b, and the 
corresponding cylinder volume passed through by the piston repre- 
sents the percentage of loss of volumetric capacity as stated above, f 
It may be added, however, that this reduction of capacity, although 

* Compressed Air, Dec, 1896, p. 151. 

t In a recent form of the Leyner compressor, the clearance volume of a 
twenty-two-inch cylinder is 1.02 per cent, of the cylinder volume. This would 
make the cb distance extremely small. 



64 



COMPRESSED AIR PLANT FOR MINES 



a matter of considerable importance in the operation of the com- 
pressor, does not involve a corresponding loss of useful work. 
The compressed air remaining in the clearance space helps to 
overcome the inertia of the moving parts at the beginning of the 
return stroke, and to compress the air on the other side of the 



250 



225 



200 



175 



150 — 



125 -i 



100 



50 



25 



I 


1 






















J 






















/ 




















o| < 

S3 « 

I */ 


7 < 

h ^ 

if 


7 


















1 #/ 








- 










^f ~^h 


***/ 


k/ f 

7 ty 


<2j/ sy / 

dy // 














/ 




/ ^ 


















// 






















\l//, 






















jr 























40 



45 



50 



10 15 20 25 30 35 

Per Cent of Piston Displacement 

Fig. 38. 



piston. A part of the work expended in compressing the clearance 
air is thus recovered. It has been observed that the clearance air 
cools slightly during the momentary stoppage of the piston as the 
stroke is reversed, but the consequent reduction of pressure is a 
negligible quantity. In expanding behind the retreating piston, 
however, the clearance air rapidly gives up its heat and does not, 
therefore, tend to raise the temperature of the incoming atmos- 
pheric air. 



DRY COMPRESSORS 



65 



The effect of piston clearance in reducing the capacity of a dry 
compressor is shown clearly by the diagram, Fig. 38, which is 
reproduced here by permission from Engineering News, May 
30th, 1 901. It shows that, for clearances above one per cent, the 
loss becomes serious even at pressures of seventy-five to one hun- 
dred pounds. 

Fig. 39 indicates the method of reducing the clearance for' or- 
dinary pistons, by casting a recess in the cylinder head to receive 
the projecting piston nut at the end of the stroke. The loss of 
volumetric capacity due to clearance of course increases with 
the air pressure, and in some compressors the piston is run exceed- 





Fig. 39. 



Fig. 40. 



ingly close to the cylinder head. When this is the case the com- 
pressor must have careful attention, so that if the working length of 
the connecting rod should be varied in fitting new brasses, the pis- 
ton will not be in danger of striking the cylinder head. 

The Johnson compressor, made in England, has an ingeniously 
designed piston (Fig. 40) to meet the difficulty just mentioned. It 
is composed of two disks, c and d, mounted on a brass sleeve, 
screwed on the piston-rod, h, and held in place by collar and lock- 
nut. The disks are so cast as to leave between them a recess, in 
which is placed a heavy helical spring, /. This spring is compressed 
sufficiently between the disks to prevent it from being further 
5 



66 COMPRESSED AIR PLANT FOR MINES 

compressed under the maximum working air pressure, but the 
clearance at the ends of the stroke is extremely small, and should 
the piston strike the cylinder head the spring gives slightly and an 
injurious shock is avoided.* 

A number of other devices have been adopted for overcoming 
the disadvantages of piston clearance. Two examples may be 
given : 

i. Longitudinal bye-pass grooves (BB) are cast in the inner 
surface of the cylinder near the ends, Fig. 39, so that when the 
piston reaches the end of its stroke a part of these grooves is un- 
covered, and the compressed air in the clearance space passes to 
the other side of the piston. 

2. In slide-valve compressors the valve may be provided with 
a so-called "trick-passage." At the end of the stroke this passage 
is brought into connection with two small ports entering the ex- 
treme ends of the cylinder. Through these passages the high- 
pressure air in the clearance space is released into the other end 
of the cylinder. 

Although by these methods the released air becomes of direct 
benefit, there is a decided objection to their employment if all the 
confined air be allowed to pass over, because the heavy pressure 
on the piston is suddenly removed, and there is a shock to the mov- 
ing parts which is clearly evidenced by pounding at the end of the 
stroke. In the most recent forms of compressor made in the 
United States the clearance space is very small, but the air con- 
fined in it is not released. 

Dry Versus Wet Compression 

Up to about 1885 there seemed to be little doubt among me- 
chanical engineers that the wet compressors were, upon the whole, 
superior to the dry, because by bringing the air into direct contact 
with water the heat is most effectually absorbed. This view is 
correct so far as heat loss alone is concerned, provided the water 
introduced into the cylinder is properly applied, as pointed out in 

* Bjorling, Colliery Guardian, Aug. 7th, 1896, p. 272. 



DRY COMPRESSORS 



67 



Chapter IV. Without cooling the percentage of work converted 
into heat during compression, and therefore lost, is as follows : 



Compression 


to 


2 atmospheres, 


9.2 % loss 


tt 




3 


15-0 % " 


a 




4 


19.6 % " 


a 




5 


21.3 % " 


it 




6 


24-0 % " 


a 




7 


26.0 % « 


11 




8 


27.4 % " 



In well-designed dry compressors, working at a pressure of 5 
atmospheres, the heat loss is reduced about one-half, or from 
21.3 per cent, to 11 per cent. Frequently, however, in ordinary 
mining practice, with single-stage compressors, the loss is fully 
15 per cent. By spray injection this loss has been cut down in the 
best American practice to as little as 3.6 per cent.,* and in some 
of the large, slow-running European wet compressors to 1.6 per 
cent. But the question of heat loss is not the only consideration. 
Low first cost and simplicity of construction are often more ad- 
vantageous than a close approximation to isothermal compression. 
Latterly the wet systems have lost ground, and it is probable that 
no wet compressors are now being built in the United States. In 
Europe also dry compression has grown in favor, at least for min- 
ing plants and others of moderate size. The matter may be con- 
sidered from two standpoints, as regards: 

1. The effect of injected water upon the compressed air and 
the machines using it. 

2. The effect of the water upon the working of the compressor. 
In addition, it is necessary to take account of the relative efficien- 
cies of the two types, but this will be deferred until later. 

First, it is unquestionable that by using large slow-speed en- 
gines, and an abundance of injection water, the air is well cooled, 
though at a higher first cost for plant. Wet compression gives a 
good indicator card. It is shown by Table IV that in compressing 
moist air somewhat less work is expended than for dry air. This 



* As stated regarding the old Ingersoll injection compressor, by W. L. Saunders, 
Compressed Air Production," p. 24. 



68 



COMPRESSED AIR PLANT FOR MINES 



is due to the fact that the specific heat of watery vapor is about 
twice that of dry air; therefore in the presence of moisture more 
heat is required to raise the temperature of the air in the com- 
pressing cylinder, and the loss of work from this cause is reduced. 

Table IV 



Absolute 
Pressure. 


Gauge 

Pressure. 

Pounds. 


Foot Pounds of Work Required to Compress 
One Pound of Air. 


Atmospheres. 


Dry Compression. 


With Sufficient 
Moisture. 


i 

2 

3 
4 

5 
6 

7 


o 

14-7 
29.4 

44-i 
58.8 

73-5 
88.2 


23,500 
37,000 
48,500 
58,500 
67,000 
75,ooo 


22,500 

35,ooo 
45,ooo 
5 2 ,5oo 
60,000 
66,000 



Theoretically, a corresponding economy takes place also when 
the air is expanded again in the machine using it. 

Notwithstanding these advantages, several serious objections 
became apparent in the use of the wet system of compression. 
Other things being equal, the amount of heat given up during 
compression is proportional to the difference of temperature be- 
tween the air taken into the cylinder and the injected water, and to 
the time of contact between the air and water. Under ordinary 
circumstances this difference of temperature is zero at the beginning 
of the stroke, reaching its maximum at the end. It follows: (i) 
that to attain a fair approach to isothermal compression the piston 
speed must be very slow; (2) that during the first part of the stroke 
but little heat is removed, and it is only when compression is com- 
plete, and the air begins to pass from the cylinder through the 
discharge valves, that the cooling effect is at its maximum. At 
ordinary piston speeds, therefore, a large proportion of the total 
heat must be given up after the discharge valves have opened ; in 
other words, after compression is completed. For this reason it 
would appear that, so far as economy of work is concerned, the 
lower final temperature due to spray injection is in a measure de- 



DRY COMPRESSORS 69 

ceptive. The warmth of the air at discharge augments its moisture- 
carrying capacity, and though it is intended that the separation of 
the water shall be as complete as possible in the air receiver, still 
it must of necessity be imperfect in a receiver of any reasonable 
size. Much moisture passes into the air mains, and deposits as 
the air cools down in long lines of piping. In cold weather it may 
freeze so as to reduce the effective diameter of the pipe. The mois- 
ture remaining in the air has a further ill effect when it is used. 
At the instant of exhaust by the drill, or other air engine, the in- 
tense cold produced by expansion causes the formation of trouble- 
some accumulations of ice in the exhaust passages. 

As to the dry compressor it must be admitted that as air is a 
poor conductor of heat it has little opportunity to give up its heat 
of compression between the strokes of the piston. Besides this, 
the piston, as it advances, rapidly covers the jacket-cooled sur- 
face of the cylinder. However, although atmospheric air as taken 
into the compressor always contains moisture, which will make its 
appearance as frost at the exhaust of the air machine, still there is 
not enough of it to cause serious trouble.* The delivery of warm 
air by a dry compressor is far less objectionable than warm air 
from a wet compressor. 

Second, as to the effect of injected water upon the working of 
the compressor. Under the best of circumstances water in the air 
cylinder is objectionable, because it makes lubrication difficult, 
causes rust, and increasing the wear of piston and cylinder in- 
volves greater expense for repairs and renewal of parts. No sat- 
isfactory method has ever been devised for lubricating the inner 
surface of wet compressor cylinders. This is one of the chief diffi- 
culties with wet compressors, and becomes most serious when the 
water is impure or gritty. It must, of course, contain no trace of 
acid, such as is often present in mine water. Water that is com- 

* The quantity of moisture in the atmosphere, or its humidity, varies with the 
climate, the season of the year, and in a measure with the altitude above sea-level. 
It is usually greatest near the ocean or any large body of water. What is commonly 
called dry atmospheric air contains from forty to fifty per cent, of the quantity neces- 
sary to saturate it. The degree of saturation in summer often reaches ninety per 
cent, or more. 



^ COMPRESSED AIR PLANT FOR MINES 

paratively harmless for use in jackets might be decidedly injuri- 
ous to the finished surfaces of working parts. It has been stated 
by Mr. W. L. Saunders that, although the thermal loss is higher 
in dry than wet compressors, the frictional loss in the moving 
parts is considerably higher in the wet compressor. The net 
economy of the best wet compressors is probably no greater than 
that of the best American dry compressors. 

It is urged on behalf of wet compression that the piston-clear- 
ance space is filled with water, and the capacity of the compressor 
is therefore increased. While this is true, yet, as water is in- 
compressible, and as a part of it must be forced out through 
the discharge valves at each stroke, the wet compressor is com- 
pelled to work in a measure like a water pump. Furthermore, 
closer attendance is required to regulate the water supply. The 
drip cock at the bottom of the receiver" must also be watched more 
closely to prevent flooding, and there is the disadvantage of having 
an injection pump to care for and regulate. 



CHAPTER VI 

COMPOUND OR STAGE COMPRESSORS 

Compound or stage compressors have two or more air cyl- 
inders, between which the total work of compression is divided. 
The air cylinders are placed tandem on a common piston-rod, as in 
straight-line machines, or respectively tandem with the steam cyl- 
inders in the duplex type. In two-stage compressors air at at- 
mospheric pressure is taken into the large or low-pressure cylinder; 
is there compressed to a certain point, and is then forced into the 
smaller or high -pressure cylinder, where it is brought up to the 
required tension (see Fig. 7). Manifestly, the size of the low- 
pressure or intake cylinder determines the capacity of the com- 
pressor. In a certain sense, the operation of a two-stage com- 
pressor is the reverse of that of a compound steam engine. 

The theory and application of stage compression are readily 
comprehended. Since the heat of compression increases with the 
air pressure produced — though not proportionately, as has been 
shown — it follows that the higher the pressure the more difficult 
does it become to keep down the temperature to a point permitting 
efficient operation of the compressor and proper lubrication of the 
air cylinder. In attempting, with a single-cylinder dry compressor, 
to compress even to 90 pounds gauge, the theoretical final cylinder 
temperature becomes 459 F., and at 100 pounds gauge 485 F. 
Though some heat is dissipated by radiation, the actual working 
temperatures corresponding to these pressures are still too high 
to be dealt with effectually by the ordinary water-jacket, because in 
a single cylinder the superficial area to which cooling can be applied 
is too small relatively to the volume of air, and the total compression 
period too short. Even when working at moderate piston speeds, 
say, not over 350 to 400 feet per minute, the cooling is very 

71 



72 COMPRESSED AIR PLANT FOR MINES 

imperfect. The compressed air, as discharged from the cylinder, 
is still hot, so that considerable loss of pressure and of work, due 
to subsequent cooling, are inevitable. 

These disadvantages are in large measure overcome by the 
adoption of stage compression, and, in view of the fact that this 
system was introduced over twenty-five years ago, it would appear 
strange that until quite recently it has been neglected, by nearly 
all compressor builders, for the ordinary pressures used in mining, 
tunnelling, and similar work. 

Formerly it was customary to employ stage compression 
only when high pressures were required, such as for pneumatic 
locomotives, riveting machines, presses, compression of gases, 
pneumatic guns, etc. For such service stage compression is in- 
dispensable; and the higher the pressure the greater becomes the 
necessity for compounding the air cylinders and the comparative 
efficiency of the system. To produce very high pressures, of 500 
to 1,000 pounds or more, three- and four-stage compression is 
employed. 

But it is now generally recognized that two-stage compression 
when properly applied presents some advantages even for press- 
ures of seventy to eighty pounds, as commonly adopted for ma- 
chine drills and ordinary air engines. The cooling during com- 
pression is more thorough because the total heat generated is 
divided between two or more cylinders. In each cylinder the tem- 
perature is lower than when the same total pressure is produced in 
a single cylinder, and the combined water-jackets afford a much 
larger cooling surface. 

A further cooling is effected by an "intercooler," placed be- 
tween the cylinders. This constitutes one of the most important fea- 
tures of stage compression. It is an intermediate cooling-chamber, 
through which the partially compressed air from the intake or low- 
pressure cylinder passes on its way to the high-pressure cylinder. 
The temperature of the air is here reduced, so that when the 
high-pressure piston begins its work the temperature of the volume 
of air on which it acts is considerably below that at which the air 
was discharged from the low-pressure cylinder. Obviously, the 



COMPOUND OR STAGE COMPRESSORS 73 

total reduction of temperature effected depends on the volume 
of the air under compression, the area of the cooling surfaces and 
the length of time the air is in contact with these surfaces; or, in 
other words, on the piston speed. The construction of the inter- 
cooler will be taken up later. 

It should not be inferred from what precedes that stage com- 
pression per se is always applicable, nor that it is necessarily more 
economical than compression in a single cylinder. Concerning 
this, several fairly well defined, though interrelated statements 
may here be made : 

i . Although stage compression is theoretically advantageous for 
all pressures, it becomes of doubtful utility for gauge pressures of 
much less than seventy-five pounds, because of the small saving 
as compared with the greater first cost and running expenses of 
the more complicated mechanism. It is generally applicable for 
pressures higher than seventy to seventy-five pounds. 

2. Stage compression is specially useful for large compressors, 
in which the percentage of saving will represent an amount suffi- 
cient to warrant the greater first cost of plant. 

3. The higher thermodynamic efficiency of stage compression is 
in some degree offset, and in poorly designed plants may be entirely 
neutralized, by the increased frictional losses involved in the use of 
several cylinders. In other words, when employing stage com- 
pression, advantage should always be taken of the opportunity 
to use a well-designed, economically working steam end, together 
with large and efficient cooling arrangements for the air end. 
If these requirements be not fulfilled, stage compression may easily 
cost more per cubic foot of air delivered than simple compression 
by a properly designed compressor. 

Almost all stage compressors are double-acting ; that is, on each 
forward and back stroke air is taken into the cylinders on one side 
of the piston, while compression and deliver}- are going on on the 
other side. The operation of the single-acting form, occasionally 
employed, will be considered first. It is materially different from 
that of the double acting compressor, but its description will aid 
in setting forth the subject of stage compression. 



74 COMPRESSED AIR PLANT FOR MINES 

Single-Acting Two-Stage Compressor. Supposing the intake, or 
low-pressure, cylinder to be filled with free air just taken in, the 
advancing piston compresses the air until a point somewhat beyond 
half stroke is reached. At this point the delivery valves open, and 
during the remainder of the stroke the compressed air, at, say, 
thirty to thirty-five pounds pressure, is being forced out through 
the connecting pipe and passages into the second or high-pressure 
cylinder. Meanwhile, no work is being done by the high-pressure 
piston. On the return stroke the air at the low pressure which 
was delivered into the high -pressure cylinder is compressed to 
the required final tension and discharged. During this return 
stroke no work is done in the low-pressure cylinder, except that 
another charge of free air is drawn in. Thus, the intake stroke 
of the low-pressure cylinder is the compression and delivery stroke 
of the high-pressure, and vice versa. During the low-pressure 
intake stroke the portion of partly compressed air remaining in 
the pipe or passage connecting the cylinders is unaffected, as it is 
shut off from both cylinders by the valves at either end. At the 
beginning of the return stroke of the high-pressure cylinder the 
air in the connecting pipe begins to flow into this cylinder, and its 
pressure diminishes according to the relative volumes of pipe and 
cylinder. In the mean time the air is being compressed in the 
low-pressure cylinder, and when its tension exceeds that in the 
connecting pipe (that is, at, say, half stroke) it begins to pass 
through the delivery valves into this pipe. During the remainder 
of the stroke the low-pressure piston is in reality acting upon 
and compressing, not only the air in its own cylinder, but also 
that which is in the connecting pipe and high-pressure cylinder. 

A serious disadvantage of the single-acting, two-stage compress- 
or of this form is that the net resistances in the two cylinders are not 
equalized. Although the actual work of compression is designed 
to be the same in both cylinders, equalization of the resistances 
throughout both strokes is practically impossible because, in the 
second half of the forward stroke of the intake piston, the air de- 
livered by it acts as a back pressure on the high-pressure piston, 
which is travelling in the same direction. This back pressure, in 



COMPOUND OR STAGE COMPRESSORS 75 

turn, assists the movement of the low-pressure piston during its 
compression stroke. In this stroke, therefore, less total resistance 
is presented than during the compression stroke of the high-press- 
ure piston. It has been pointed out by Mr. Frank Richards that, 
"to decrease the diameter of the high-pressure cylinder would 
tend toward equalization of the resistances, by allowing the intake 
cylinder to do more work, and compress the air to a higher press- 
ure; but to raise the pressure (at delivery) in this cylinder would 
be to defeat the object of two-stage compression — that of allowing 
an efficient cooling of the air, and a reduction of its volume before 
its compression is too far advanced." In stage compression it is 
a fundamental principle that the cylinders should be so propor- 
tioned that the total work is divided equally between them. This 
secures the largest saving possible in the mechanical work of the 
compressor, as well as in efficiency of the cooling apparatus. 

Double-Acting Two-Stage Compressors. The operation of this 
type is more satisfactory than that of the single-acting two-stage 
compressor, because, first, the cycle of operations during each 
forward and back stroke is the same; and, second, the distribu- 
tion of the resistances throughout the stroke may be made more 
uniform. 

A number of combinations in the arrangement of the steam and 
air cylinders are possible, but three forms only need to be noticed, 
as representing accepted practice, viz.: the straight-line, two-stage 
compressor (Figs. 7 to 11) and the duplex forms, consisting of a 
pair of cross-compound air cylinders, placed tandem to either 
twin, simple, or cross -compound steam cylinders (Figs. 13 to 19). 
The last-named is undoubtedly the best for large plants. 

The principles of the mode of operation of all three designs may 
be illustrated by reference to Fig. 41, which shows diagrammat- 
ically a Norwalk two-stage straight -line compressor. 

Assuming that the pistons have reached the end of their forward 
stroke, the conditions in the two cylinders are approximately as 
follows: The low-pressure cylinder (D) is full of air, practically 
at atmospheric pressure, while the high-pressure cylinder (G), to- 
gether with the intercooler (F) and connecting passages, are oc- 



76 



COMPRESSED AIR PLANT FOR MINES 




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COMPOUND OR STAGE COMPRESSORS 77 

cupied by air just delivered from the low-pressure cylinder, at, say, 
forty pounds, or about one-half the final pressure. On the reverse 
stroke the free air in front of the low-pressure piston is com- 
pressed to forty pounds and delivered into the intercooler and high- 
pressure cylinder, while the air already occupying the latter is 
brought up to the final pressure and discharged. This must be 
considered only as a rough description of what takes place in the 
air cylinders during a complete forward and back stroke. 

As usually constructed for standard tandem, two-stage com- 
pressors, the volumetric, capacities of the low- and high-pressure 
air cylinders are to each other in the ratio of about ten to four. 
The intention is to proportion the two cylinders so that their 
ratios of compression are nearly equal.' Thus the distribution 
of work and the heat generated in the cylinders will be equalized 
and most effectually dealt with by the intercooler, provided 
the latter properly performs its functions. Practice as regards 
the relative volume of the intercooler and cylinders has not 
yet been completely standardized. It has undergone consid- 
erable change in the past few years. As clearer conceptions 
have been reached of the fundamentally important functions of the 
intercooler in stage compression, and in recognition of the fact 
that the first cost of even a very large intercooler is moderate, while 
its running expenses are practically nil, the tendency now is to make 
it of much greater volumetric capacity than formerly. Such in- 
crease of size produces substantial gain in thermodynamic efficiency. 
The hot compressed air delivered by the low-pressure cylinder is 
kept longer in contact with the cooling surfaces because of its 
reduced speed of flow through the larger cross-sectional area of the 
intercooler, and it enters the high-pressure cylinder, to undergo the 
second stage of compression, with a temperature that may readily 
be made to approximate closely to the normal. On the other hand, 
it is clear that the connecting passages between the cylinders and 
intercooler should be of as small volume as is consistent with free- 
dom from excessive frictional resistance in the flow of the air 
through them ; because the air occupying these passages at any given 
time is exposed to but little cooling save that due to radiation. 



78 COMPRESSED AIR PLANT FOR MINES 

With these points in view, it may be assumed in good practice 
that, if the volume of the low-pressure cylinder be taken as 10, then 
the volume of its connection with the intercooler should be, say, 1.5, 
of the intercooler 4, of the connection to the high-pressure cylin- 
der 1.5, and of the high-pressure cylinder 4. (It may be noted 
that there is no reason why the net capacity of the intercooler 
should not be even greater than is here assumed.) Having these 
proportionate volumetric capacities, the following sequence of 
operations will take place while the compressor is making a single 
stroke. Suppose this stroke to be from right to left, as indicated 
by the arrows in Fig. 41 . 

By the previous stroke (from left to right) the intercooler and 
both of its connections to the cylinders, representing a volume 
= 1.5 + 4 + 1.5, were filled with air compressed, at, say forty pounds. 
This body of air was then shut off from both cylinders by their 
respective valves, and has lost part of its heat and pressure by the 
action of the intercooler. After reversal, and during the first part 
of the following (left-hand) stroke, the low-pressure piston acts only 
on the cylinderful of free air just taken in (volume = 10).* While 
this is being compressed, the advance of the high-pressure piston 
causes the compressed air already in the intercooler and its con- 
nections to begin to flow into the high-pressure cylinder, thereby 
increasing in volume and decreasing in pressure, until a point, say, 
a little beyond mid-stroke is reached. On passing this point the 
air pressure in front of the low-pressure piston rises slightly higher 
than that in the intercooler and the corresponding low-pressure 
delivery valves open, so that the low-pressure piston acts upon the 

entire body of air — volume = hi. 5 + 4+1. 5+ — = 14. Then, 

2 2 

until the end of the stroke, both cylinders are in communication 
through the intercooler, i.e., from the left-hand end of the low- 
pressure cylinder to the right-hand end of the high-pressure cylin- 

* The general method of analysis here given is similar to that employed some 
years ago by Frank Richards, " Compressed Air," pp. 86-87, though the quantities 
used are taken to represent a closer approach to current practice in the proportions 
of the parts. 



COMPOUND OR STAGE COMPRESSORS 79 

der, as shown by the arrows in the cut, and an approximate equal- 
ization of pressure is established throughout. 

Up to the time of the opening of the left-hand, low-pressure 
delivery valves, the air in the intercooler, and still under its in- 
fluence, has been isolated from the low-pressure cylinder, in which 
compression has progressed without other cooling than that effected 
by the cylinder water-jacket. But when the warm, partly com- 
pressed air begins to pass from the low-pressure into the high- 
pressure cylinder, through the intercooler, the influence of the 
latter is exerted upon a new body of air. At the end of the left- 
hand stroke the closing of the delivery valves again shuts off the 
air in the intercooler from both cylinders. The high-pressure cylin- 
der, on the right-hand side of the piston, is occupied by a body of 
air whose temperature has been reduced by the combined effect of 
the intercooler and both water-jackets to a point much below that 
due to the working pressure of the low-pressure cylinder, and 
whose pressure has dropped correspondingly. 

Now, in the latter part of the left-hand stroke, when the low 
pressure delivery valves have opened and the piston of this cylin- 
der is acting on the volume 14, as stated above, a portion of this 

2 +1.5 
air (volume = =25 per cent.) of the total has passed beyond 

the influence of the intercooler, and another portion (volume = 
=46 per cent.) has not yet reached it. A similar statement 

of the distribution of the air with respect to the intercooler may be 
made for other points of the stroke. At the end of the left-hand 
stroke under consideration the volume of compressed air in the 
low-pressure cylinder = 0, in the intercooler and its connections 
1.5 + 4 + 1.5 = 7, and in the high-pressure cylinder 4, a total of 
11, of which 1.5 has not reached the intercooler but has been 
affected only by the low-pressure water-jacket. 

This analysis should be clearly understood in forming a correct 
estimate of the work actually accomplished by the intercooler. 
It emphasizes the importance not only of employing an inter- 
cooler whose volumetric capacity is large relatively to the cylinders, 



80 COMPRESSED AIR PLANT FOR MIXES 

but also of making the connecting passages small. It is evident 
that one-half of the total work of compression — that performed in 
the high-pressure cylinder — is done solely imder such cooling in- 
fluence as may be exerted by the water-jackets of this cylinder. 
The jackets of both cylinders should, therefore, be as large in area 
as possible, with an efficient circulation of cold water. They 
should cover not merely the cylinder barrels, but as much of the 
heads as the spaces occupied by the valves will permit. In the 
latter respect some recent compressor designs are deficient. 

The details of the distribution of the air in the foregoing de- 
scription apply exactly only to compressors in which the air cyl- 
inders are tandem to each other. In the duplex stage-compressors, 
where the air cylinders usually are, and always should be, cross- 
compounded, the cycle of operations is different because the pis- 
tons, instead of moving together in the same direction, work with 
one crank 9c in advance of the other. 

As stated above, it is intended in stage compression that the 
total work done shall be equally divided between the air cylinders. 
But, by reason of the frequent variations in receiver pressure, up- 
on which depends the actual terminal pressure of the high-press- 
ure cylinder, an approximate equalization only can be attained 
in practice. On the basis of some terminal pressure taken as 
normal, such diameters are assigned to the cylinders as will make 
their compression ratios equal, or nearly so. Take, for example, 
a pair of cylinders, 15 ins. and 24 ins. in diameter, to produce 
a final pressure of 85 lbs. gauge. Assuming that the air between 
the stages is cooled to the original temperature, the absolute in- 
take pressures of the cylinders will be inversely proportional 
to the squares of their diameters, or: 15 2 : 2_j 2 : : 14.7 : 37.64. 
The absolute pressure of 37.64 lbs., as delivered by the low- 
pressure cylinder, is theoretically equal to the intake pressure of 
the high-pressure cylinder. The ratio of compression in the low- 
pressure cvlinder is: — r-=°-39°5; and in the high-pressure 

37.64 3 " 4 

cvlinder: — — =0.377;. This would be quite as close to per- 

99-7 
feet equalization as is necessary. 



COMPOUND OR STAGE COMPRESSORS 



8l 



Construction of the Intercooler. A number of forms are now 
in use. As commonly constructed for straight-line compressors, 
the intercooler consists of a long cylindrical chamber, containing 
a number of parallel, thin brass (sometimes wrought-iron) tubes, 
through which cold water is circulated. The air to be cooled 
passes through the spaces between the tubes. The intercooler 
is placed in a convenient position between and above the cylinders, 
and as close to them as possible, so that the connecting passages 
may be short and of small volume. As already stated, the air con- 
tained in these passages at any given time is denied the cooling 




Fig. 42. — Horizontal Intercooler. Ingersoll-Rand Co. 

effect both of the cylinder water-jackets and of the intercooler 
itself. In Fig. 41 the intercooler is indicated at F; in Fig. 7 the 
Norwalk intercooler is shown in longitudinal section. Fig. 42 
illustrates a large horizontal intercooler, as built by the Ingersoll- 
Rand Co. Another design, for cross-compound air cylinders, by 
the Sullivan Machinery Co., is shown in Fig. 43, a large intercooler 
being placed crosswise below the cylinders. In many cross-com- 
pound compressors the intercooler is mounted above the cylinders. 
The tendency now is to increase the size and volume of the inter- 
cooling chamber, relatively to the volume of the cylinders. 

The air delivered from the low-pressure cylinder passes on its 
way to the high-pressure cylinder between the intercooler tubes, 
which must be sufficiently close together thoroughly to split up the 
body of air traversing the intermediate spaces and so secure the 
maximum cooling effect. It is intended that the temperature of 
6 



82 



COMPRESSED AIR PLANT TOR MINT 




COMPOUND OR STAGE COMPRESSORS 83 

the air, on leaving the intercooler and entering the high- pressure 
cylinder, shall be reduced nearly to the normal. The effect of this 
drop in temperature upon the compression curve of a two-stage 
compressor is shown by Fig. 46; the curve of the high-press- 
ure cylinder should, and often does, begin close to the iso- 
thermal line. 

In the construction of the intercooler brass tubes are perhaps 
preferable to those of iron because of their higher conductivity; 
but, on the other hand, iron tubes cost less, and on account of their 
greater roughness present a larger cooling surface to the air flow- 
ing between them. In either case they should be as thin as is con- 
sistent with the necessary strength. The tubes are expanded into 
tube-sheets at each end, and by means of two or more baffle- 
plates, set equidistant between the ends, the air is compelled to 
pass through the entire volume of the intercooler. The water- 
heads at the ends are so divided that the water is caused to circulate 
actively back and forth several times, before it is finally discharged, 
as shown by the small arrows in Fig. 42. For convenience the 
water supply is usually connected with the circulating system of the 
cylinder- jackets. 

Fig. 44 illustrates a peculiar system of intercooling adopted in 
the Leyner compressor. A number of horizontal iron or bronze 
tubes are enclosed in the annular water-jacket spaces, between the 
inner and outer shells of the cylinder. The piston being at the 
middle point of its stroke, the inlet valves at the left-hand end of 
the low-pressure cylinder are open and taking in air. Meantime 
the air in front of the piston, having been compressed, is passing 
out through the delivery valves into the air chamber or head at 
the right-hand end of the cylinder. This air is thence forced by 
horizontal baffle-plates in the air chamber through the upper set 
of intercooler tubes, and into the left-hand end of the cylinder. 
It flows next to the right, through the lower set of intercooler tubes, 
and as shown by the arrows enters the lower tubes of the high-press- 
ure cylinder. From the right-hand air head of this cylinder the 
air is directed by baffle-plates back through the upper set of tubes to 
the left-hand end of the high-pressure cylinder, into which it enters 



84 COMPRESSED AIR PLANT FOR MINES 

through the corresponding inlet valves. The air already com- 
pressed in this cylinder is shown as passing through the large upper 
aftercooling tubes to its own air chamber, which leads to the dis- 
charge pipe. It will be noted that the low-pressure air, in being 
subdivided into small volumes and compelled to change its direc- 
tion several times in passing back and forth through the intercooler 
tubes, is well cooled before entering the high- pressure cylinder. 
It is important that the copper tubes of the intercooler be kept 
clean. As the oil carried over by the air tends to deposit on the 
tubes, they should be so arranged as to be readily accessible for 
cleaning. The intercooler of the Schram (English) two-stage com- 
pressor is a vertical chamber, also rilled with small tubing. The 
water enters at the bottom, passes up through one-half of the tubes 
and down through the other half, the lower water-head being di- 
vided accordingly. The air from the low-pressure cylinder enters 
at the top of the intercooler, passing out at the bottom into the 
high-pressure cylinder. 

Although the relatively small intercoolers of ordinary two-stage 
compressors are imperfect in their action, as has been pointed out, 
it is nevertheless possible to attain a high degree of efficiency from 
intercoolers of large capacity. A well-known example may be 
cited: the plant of the Paris Pneumatic Supply Co., in which 
Riedler two-stage compressors are used. Spray injection is applied 
to both cylinders, and also a plain intermediate receiver of very 
large capacity, but without tubes. The air is compressed to 88 
pounds, and the indicator diagrams of the air cylinders exceed in 
area the true isothermal diagram by only 12.07 per cent.* That 
is, the work done twice is about 12 per cent, of the total work, the 
total efficiency having the high value of 77 per cent. 

To show the results obtained by thorough cooling of the air 
between the cylinders, a comparison of the work done by single- 
and double-stage compression may be made. Frictional losses 
will be omitted in each case, and no account will be taken of the 
cooling due to the cylinder water-jackets. 

1. A single-stage compressor, producing a gauge pressure of 

* Proceedings Institution of Civil Engineers, London, Vol. CV, p. 180. 




he 

c 



X 
m 



c 






86 COMPRESSED AIR PLANT FOR MIXES 

70 pounds at sea-level, with a 24-inch cylinder and a piston speed 
of 400 feet per minute, will have a capacity in terms of free air at 
normal temperature of 1,256 cubic feet per minute. For adiabatic 
compression, the mean cylinder pressure will be 33.83 poimds and 
the horse-power 184.38. 

2. For doing the same work in a two-stage compressor, provided 
with an intercooler capable of reducing the temperature of the air 
to the normal between the cylinders, it may be assumed that the 
low-pressure or intake cylinder has the same diameter, 24 inches, 
and that the pressure produced in it is 35 pounds. The mean press- 
ure (adiabatic), corresponding to 35 pounds terminal pressure, is 
21.6 pounds, and the horse-power 11 8. 19. The diameter of the 
high-pressure cylinder, under the assumed conditions, is found by 
making the piston area inversely proportional to the increase in 
absolute pressure of the air delivered to it by the low-pressure 
cylinder, i.e., in the ratio of 14.7 : 35 + 14.7 = 1 : 3.38. This gives 
an area of 135 square inches, equivalent to 13 inches diameter. 
Compressing in this cylinder from 35 to 70 pounds gauge, the mean 
effective pressure will be 28.74 pounds, and the horse-power, 46; 
or a total for both cylinders of 11 8. 19 4- 46 = 164.19 horse-power. 

Compared with the power required for doing the same work 
in the single cylinder, this shows a saving of: 184.38 — 164.19 = 
20.19 horse-power, or about eleven per cent. The theoretically 
perfect cooling between the cylinders here assumed would not be 
attained in ordinary practice, however, and the frictional loss in 
the stage compressor would probably be a little greater than in 
the single-cylinder machine; so that the net gain due to inter- 
cooling may in this case be taken at, say, seven to eight per cent. 
The saving is considerably increased in dealing with higher 
pressures. 

The advance made in recent years in the design of intercoolers 
is further illustrated by Fig. 45, showing a new design of the 
Ingersoll-Rand Co. It is provided with pipe connections for drain- 
ing off the water deposited as a result of the reduction in tempera- 
ture. These coolers may be employed also as " receiver-after- 
coolers," which are now considered as almost essential adjuncts 



COMPOUND OR STAGE COMPRESSORS 



87 



of well-installed large plants. (See Chapter XL) A similar 
appliance may be employed advantageously as an ante-cooler for 
the intake air. 

The useful effect of small intercoolers, such as are frequently 
mounted above the cylinders of straight-line compressors, should 




34-75. 



Fig. 45. — Vertical Intercooler. Ingersoll-Rand Co. 



not be misunderstood nor exaggerated. It must be remembered 
that the best economy in air compression is obtained only by 
cooling during compression and before the air leaves the cylinder. 
Hence, in addition to the intercooler, the largest possible water- 
jacket area should be provided. 

The relation between the compression curves of a two-stage 



88 



COMPRESSED AIR PLANT FOR MIXES 



compressor is shown in Fig. 46, the adiabatic and isothermal curves 
being also laid down.* These cards, not accurately reproduced 
here, were taken from a pair of cylinders measuring 7 J and 14 X 16 
inches, compressing to no pounds gauge, at 135 revolutions per 
minute, or 360 feet piston speed. Initial temperature of cooling 
water, 55 : temperature at discharge from jackets and intercooler, 







%\ \\ \ 














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vfr 


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Line 


b 




Vacuum 







Line 



Fig. 46. — Combined Air Card of Two-Stage Compressor. 



62 ° F. Several points are to be noted in connection with these 
combined two-stage cards : 

First. The overlapping of the high- and low-pressure cards in- 
dicates a loss, because the work represented by the area of the over- 
lap is in reality work done twice. This is the result of the drop 
in pressure between the cylinders, which is caused by the resistance 
presented by the discharge valves of the low-pressure and the inlet 
valves of the high-pressure cylinder, together with the friction 
in the air passages and intercooler. While this loss is unavoidable, 

* This combined indicator card, which does not show all the minor irregularities 
in the lines, is from a Rand cross-compound compressor. It accompanies an ar- 
ticle by F. A. Halsey. on ''The Analysis of Air Compressor Indicator Diagra 
American Machinist, March 3d, 1898, p. 158, and is reproduced here by permission. 



COMPOUND OR STAGE COMPRESSORS 89 

it should be reduced as much as possible by making the valves, 
ports, and connecting passages of ample size. 

Second. As with single-cylinder dry compressors, the com- 
pression lines of the individual cylinders of most stage compressors 
depart but little from the adiabatic curve. Aside from the thermo- 
dynamic advantage of dividing the total compression between two 
or more cylinders, and thereby lowering the average and final tem- 
peratures, it is the intercooler that must be relied on for furnishing 
the chief element in economical working. By its abstraction of heat 
the volume of air entering the second cylinder is reduced, so that 
PV" =M = constant becomes approximately PV = C, on beginning 
the second stage of compression. But the compression line again 
rises rapidly from this point and continues not far below the 
adiabatic. 

Indicator cards from dry compressors which do not show 
approximately this relation between the lines are always open to 
suspicion. A leaky piston, for example, will lower the compression 
curve and make it appear that much better work is being done than 
is really the case. It may be observed that, other things being 
equal, a lower curve is often obtainable from a small than from a 
large compressor, because the ratio of area of water-jacket to the 
volume of the cylinder is greater. 

In constructing and reading a combined indicator card from 
both cylinders of a stage compressor (like that shown in Fig. 40), 
the adiabatic line applying to the compression in the second cylinder 
should be represented in its proper place. The complete graphic 
relation between the several heat curves is thus set forth . 

Third. It is an advantage of stage compression that there is 
practically but one clearance space — that in the low-pressure cylin- 
der — and, as the air in this cylinder is at a low pressure, the re- 
sulting reduction in net volumetric capacity is moderate, for it is 
evident that the loss due to clearance is proportionately less for low 
than for high pressures. The piston clearance of the high-pressure 
cylinder cannot affect the volume of air delivered, because all the 
air discharged from the low-pressure cylinder goes to the high- 
pressure and, barring leakage, must pass through it. 



9° 



COMPRESSED AIR PLANT FOR MINES 



The heating of the cylinder walls and pistons reduces somewhat 
the working volumetric capacity of an air compressor because, as 
the entering air is warmed, a smaller weight of it is taken into 
the cylinder at each stroke. Although the degree of this heating 
cannot be formulated, it is obvious that it is less in a two-stage 
than in a single- cylinder compressor; for, aside from the effect of 
the intercooler, the smaller quantity of heat developed in each 
cylinder is more efficiently dealt with by their respective water- 
jackets. 



CHAPTER VII 

AIR INLET VALVES* 

The proper design and working of the inlet or suction valves 
exert an important influence on the efficiency of the compressor, 
and perhaps no other one portion of air-compressor mechanism has 
received so much attention. Nevertheless, that there are still 
wide differences of opinion as to the best design for inlet valves 
is evidenced by the great variety of types used by compressor- 
builders and the lack of clearly defined distinctions as to their 
applicability under different working conditions. Reference to 
almost any compressor catalogue will show that the purchaser has 
a choice of several types, with but little to guide him in making a 
selection. 

In the older forms of wet compressor various patterns of clack- 
valve were employed, as exemplified in the Dubois-Francois com- 
pressor. Though not now used in this country, they have by no 
means been abandoned in Europe; witness the Guttermuth valve 
and the elaborate, cam-controlled clack-valves of some large com- 
pressors built by Schneider & Co., Creusot, France. For years 
poppet valves of numerous types held the field in the United States 
almost exclusively. They are furnished with springs, and are usu- 
ally actuated solely by difference of air pressure ; though in a few 
designs mechanically controlled poppets were introduced, such as 
those of the old Rand mechanical valve-gear and others, examples 
of which are still occasionally to be found in use. While poppet 
valves have continued in favor for certain kinds of service, and are 
likely to remain so, many other forms of inlet valve have been 

* This chapter is devoted chiefly to spring poppet valves and others which 
operate by difference of air pressure. For discussion of those inlet valves whose 
movements are under mechanical control, see Chapter IX. 

9i 



92 COMPRESSED AIR PLANT FOR MINES 

successfully applied in the course of the development of the 
modern compressor. Modifications of the Corliss rotary steam 
valve, first used in the Norwalk compressor, have now been 
adopted in compressors of many other makes, such as the Nord- 
berg, Sullivan, Laidlaw-Dunn-Gordon, and Allis-Chalmers. There 
are at least two inlet valves which cannot be included in any of the 
other classes, viz. : the Sturgeon valve, placed in the cylinder head 
and operated by frictional contact with the piston rod, and the 
ingenious Ingersoll- Sergeant piston inlet, which opens and closes 
by its own inertia at the end of each stroke. Both of these operate 
under fixed conditions, independently of differences in air pressure 
within and without the cylinder. 

The two chief requisites of all inlet valves are: i. That they 
shall have a sufficient area of opening to permit free entrance of 
the air. 2. That they shall open readily near the beginning of the 
stroke, with a minimum of resistance, remain open until the end 
of the stroke, and then close promptly. 

There are several questions affecting the design and operation 
of the usual types of inlet valve, which are closely related to the 
working of the air cylinder itself. The point of the stroke at which 
the inlet opens should depend on the piston clearance and the 
air pressure under which the compressor is working. Spring-con- 
trolled valves, or those operated mechanically, are sometimes in- 
correctly designed or set, so as to open exactly at the beginning 
of the stroke or a fraction later; in which case the clearance air 
is first exhausted through the valves and then, as the piston ad- 
vances, the outside air begins to enter. This being so, it is evident 
that no clearance at all would be shown on the indicator card. 

As already pointed out, although piston clearance causes a 
reduction in volumetric capacity of the cylinder, it not only does 
not involve a corresponding loss of work, but is in reality beneficial, 
in assisting to overcome the inertia of the reciprocating parts of the 
compressor. A large part of the work expended in compressing 
the clearance air is thus recovered. But when the clearance air is 
exhausted wholly or in part by a premature opening of the inlet 
valves, the work represented by it is lost. With spring-controlled 



AIR INLET VALVES 93 

poppet valves the proper adjustment is a question of the strength 
of the spring, and since the effect of clearance varies with the air 
pressure, the valves must be regulated for the pressure carried in 
each particular case. Any exhaust through the inlet valves is 
readily detected by the noise. When they are properly set, the com- 
pressor works more smoothly and the power consumed is slightly 
reduced. On the other hand, if the valves open too late in the 
stroke — due, for example, to a temporary reduction in working 
pressure — a little more power is required, this condition being 
shown by the slight drop in the re-expansion line at the point b 
(Figs. 37 and 46). 

For inlet valves which are opened and closed mechanically, 
an adjustment to the working conditions is even more imperative 
than in the case of valves controlled only by springs. If incorrectly 
set or timed with respect to the stroke of the piston, they may be 
forcibly opened too early in the stroke or closed before the end. 
Premature closing obviously reduces the volume of intake air, and 
with it the volumetric capacity of the compressor. Its effect on the 
indicator card is to lower the compression line near the beginning 
of the stroke, so as to approach the isothermal curve and make 
it appear that the compressor is doing abnormally good work. 

The total area of the inlet ports varies greatly in compressors of 
different makers. It is sometimes as small as 3 or 4 per cent, 
of the piston area, running up to a probable maximum of 
12 to 14 per cent. As the proper area is really a function 
of the piston speed, it may be made less for slow- than for 
high-speed compressors. However, in one of the Leyner 2- 
stage compressors, with a 2 2 -inch low-pressure cylinder and run- 
ning at the moderate piston speed of 390 feet, the intake port area 
is 14.2 per cent, of the piston area. The valves are of a special 
type, described hereafter. To insure freedom from excessive fric- 
tional resistance against the inflow of air, the inlet area, under 
average conditions and for ordinary forms of valve, should be not 
less than, say, ten per cent, of the piston area. But extremes should 
be avoided. If poppet valves are made unnecessarily large, 
their inertia becomes too great; and if too numerous, there are not 



94 



COMPRESSED AIR PLANT FOR MINES 



only more parts to care for, but valuable water-jacket area on the 
cylinder heads must be sacrificed. 

Poppet Inlet Valves. One of the commonest forms is the 
mushroom valve, two types of which are shown in Figs. 47 and 48. 
While the total inlet area should be ample, there are two special 
requirements in the case of ordinary poppet valves: (1) the area of 
each individual valve must be moderate, or the valve will become 
too heavy, causing unnecessary injury to the valve seat, and by 
its inertia too great a resistance to the control of the spring; 
(2) the lift must be small, in order to attain prompt opening and 




1 

Fig. 47. — Norwalk Poppet Inlet Valve. 

closure, and to reduce "chattering," as well as wear. For these 
reasons the total area required is furnished by a number of in- 
dependent valves, generally from four to six, which are set in each 
cylinder head. 

The valve is of steel or bronze, with an easily removable bronze 
seat, the contact surfaces being ground true and the seating often 
coned. To control and close the valve promptly its stem is pro- 



AIR INLET VALVES 



95 



vided with a spiral spring. The stem works in guides, forming 
part of the seat and valve casing, which is screwed into the cylinder 
head so as to be readily removed when necessary for adjustment 




Fig. 48. — Laidlaw-Dunn-Gordon Poppet Inlet Valve. 

or repairs. Brass springs are used, to avoid the effects of corrosion, 
and must be easily compressible to allow the valve to open freely 
under a small difference of pressure; that is, as early in the stroke 



96 COMPRESSED AIR PLANT FOR MIXES 

as possible after the clearance air has re- expanded. The springs 
should be made of the best material and accurately proportioned to 
present no more than the minimum requisite resistance to opening. 
Under actual working conditions the pressure of the springs varies 
from, say, three ounces to eight or even ten ounces per square inch 
of valve area. 

Ordinary poppet valves are opened by the atmospheric pressure 
from without, when a certain degree of rarefaction of the air inside 
the cylinder has been produced by the movement of the piston; 
in other words, when the difference of pressure, after the clearance 
air has re-expanded, becomes sufficient to overcome the resistance 
of the spring, and compress it. In accomplishing this the piston 
must advance some distance before any air can enter the cylinder. 
The loss of volumetric capacity thus caused, in terms of free air, is 
probably rarely less than two to three per cent., and is often more. 
At sea-level a spring pressure of five ounces per square inch of 
valve area causes a loss of about two per cent. The diagram, 
Fig. 49,* shows the effect of spring resistance in reducing the volu- 
metric capacity of a compressor at different altitudes, from sea- 
level to 15,000 feet elevation. 

With spring-controlled poppets there is more or less irregu- 
larity in the entrance of the air, because, while the pressure of the 
outside air tries to open the valve, the action of the spring tends to 
keep it closed. This often produces " chattering " or ' ' dancing " of 
the valves, and has led among other things to the introduction of 
various mechanical devices for definitely controlling them, as will 
be noted later. As the springs lose their original elasticity, and 
undergo alterations in strength, they require regulation from time 
to time ; outside adjusting nuts on the valve stems may be provided 
for this purpose. If the springs be too slack, the chattering increases ; 
if too tight, the valves will open late in the stroke, and the intake air 
occupying the cylinder will have a density less than that of the 
atmosphere. But, aside from spring resistance, the rate of inflow 
of the intake air is variable. This is due to the variation in speed 
of the piston. When its speed is greatest, at the middle of the 

* Reproduced by permission from Engineering News, May 30th, 1901, p. 391. 



AIR INLET VALVES 



97 



stroke, the rate of inflow of air is at the maximum. While the pis- 
ton is moving slowly, near the beginning and end of each stroke as 
the crank turns its centers, the relatively small negative pressure 













































































































































































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9 8 



COMPRESSED AIR PLANT FOR MINES 



becomes insufficient to open the valves and keep them open against 
the strength of the springs. The effective length of the stroke is 
thus shortened. 

The total valve resistance,including that due to throttling of the 
intake air and friction in passing through the ports, must be kept as 
small as practicable, but can never be entirely eliminated. With 
some forms of inlet valves, other than spring poppets, the re- 
sistance becomes very small, and sometimes almost inappreciable. 




Fig. 50. 



Its usual effect is shown on the diagram, Fig. 50. There is generally 
sufficient resistance to keep the admission line, A C, at an apprecia- 
ble distance below the atmospheric line, D E, throughout the 
stroke; the amount of loss from this cause being measured by the 
area of the indicator diagram lying below the atmospheric line. If 
the inlet area be too small or the valves poorly designed, the result- 
ing negative pressure may amount to one or two pounds per square 
inch. The point B, where the compression line crosses the atmos- 
pheric line, is the point of the stroke which must be reached by the 
piston before any useful work is done, and the volume passed 
through in travelling from A to B represents the loss in volumetric 
capacity from this cause. The total loss of volumetric capacity, 
including that due to piston clearance, is represented by the length 



AIR INLET VALVES 99 

of A B + C E, and the volumetric efficiency of the compressor is 
measured by the length of the line B C, projected on the atmos- 
pheric line. 

Notwithstanding certain inherent disadvantages, the poppet 
valve in different forms is widely used, for both inlet and discharge. 
It is simple in construction, easily regulated, and in case of leakage, 
due to cutting and unequal wear of the seating surfaces, is readily 
removed and re-ground. In stage compressors it is sometimes 
used for the high-pressure cylinder, even when some other type is 
preferred for the low-pressure. Poppet inlet valves not infre- 
quently cause trouble by sticking in their seats on account of the 
accumulation of gummy oil. Or, they are sometimes clogged by 
deposit of carbonaceous matter from decomposition of the lubri- 
cant, produced by excessive heating of the cylinder. The valves 
should be kept clean, and are therefore designed to permit ready 
access. 

One of the recent forms of Norwalk two-stage compressor 
has a special poppet inlet valve, designed for use when it is desired 
to employ air at two different pressures, obtained from a single com- 
pressor. In stage compression, though the air is actually pro- 
duced at two pressures, of, say, 25 to 3c and 80 to 100 pounds, re- 
spectively in the low- and high-pressure cylinders, yet, if a part 
of the volume delivered by the low-pressure cylinder be drawn 
from the intercooler, the high-pressure cylinder fails to work satis- 
factorily. The air remaining in the intercooler expands to a lower 
pressure before going to the high-pressure cylinder, so that the ratio 
of compression in this cylinder is increased, and the heat gener- 
ated is raised to a correspondingly higher degree. With such a rise 
in temperature as would be produced by increasing the ratio of 
compression from, say, three to fifteen or twenty, proper lubrica- 
tion is impossible, and the conditions would be favorable for an 
explosion in the cylinder. 

This difficulty is met by using " skip-valves " (Fig. 51) as inlet 
valves of the high-pressure cylinder. They are designed to open, 
and remain open, whenever the high-pressure inlet air falls below 
the normal, by reason of having drawn off a portion of the air from 



IOO 



COMPRESSED AIR PLANT FOR MIXES 



the intercooler. The high-pressure cylinder is thus temporarily 
unloaded in part, since the air entering at each stroke is returned 



/C&s 




o 
U 



u 

o 



> 



go 



to the intercooler. The skip-valve is a mushroom spring-poppet, 
D, carried in the guides A,A. Above the valve is a small, spring- 
controlled plunger B, the space below which is occupied by air at 



AIR INLET VALVES 



IOI 



intercooler pressure. When this pressure falls below that for 
which the spring C is set, the plunger advances and forces open 
the inlet valve, holding it open until the intercooler pressure rises 
sufficiently to cause the plunger to recede. The valve is then 
free to work automatically in the usual manner. The action of 
the valve thus adjusts itself constantly to the varying pressure 
of the intake air coming from the intercooler ; and the variation 
in consumption of power by the high -pressure cylinder is taken 
care of by the governor applied to the steam end of the com- 
pressor. 

Ingersoll-Sergeant Piston-Inlet Valve. In the Ingersoll-Rand 
compressor the inlet valves are placed in the piston (Fig. 52). The 




Fig. 52. — Piston Inlet Compressor. Ingersoll-Rand Co. 



piston is hollow, and into its rear end is screwed a hollow back 
piston-rod passing out through a stuffing -box in the cylinder head. 
There are two large ring-shaped valves, made of composition 
metal, one in each side of the piston. These valves rest in their 
seats without springs or other connection, except that in the piston 
casting there are several small studs which pass through slots in 
the valve ring. While the compressor is running, the air is drawn 
in through the hollow piston-rod in an almost constant stream, 
passing through either valve first into one end of the cylinder, 
and then into the other. At the beginning and end of each stroke 
the valves are alternately opened and closed by their own inertia, 



102 COMPRESSED AIR PLANT FOR MIXES 

as the piston reverses its motion. The valve in that face of the 
piston which is toward the direction of movement is always closed, 
while the other is open for the passage of the air entering through 
the hollow rod into the cylinder behind the piston. On account 
of the large size of the valves their throw, or lift, is only about 
one-quarter inch. 

In this compressor the area of the air-inlet pipe is about ten per 
cent, of the piston area. Although the actual port area of the 
valve itself is less than this — say, six per cent. — the velocity of in- 
flow is moderate and the volumetric efficiency high. This net area 
is less than for some compressors having a group of valves, but is 
found to be sufficient because the inlet is concentrated in a single 
opening. It is probable that during admission there is less differ- 
ence between the pressure of the air taken into the cylinder and the 
atmospheric pressure than with any form of spring-controlled 
valve, for, meeting with no resistance due to springs, the air enters 
freely. Moreover, when the end of the stroke is reached the in- 
flow of air is suddenly checked, and the momentum of the column 
of air in the inlet pipe tends to cause a slight increase in the density, 
and therefore the weight, of the body of air already taken into the 
cylinder. 

These valves wear well, and their use permits a moderately 
high piston speed. Other advantages are: the cylinder castings are 
simplified ; the space in each cylinder head otherwise occupied by 
inlet valves may be utilized for additional water-jacket area ; and 
the number of moving and wearing parts is reduced. It is probable, 
however, that these advantages are partly offset by the rise in 
temperature of the intake air in its passage through the hollow 
rod and piston. These are necessarily heated, so that the weight of 
air tilling the cvlinder is relatively less than if it had entered bv a 
more direct path. 

Johnson Valve. In the Johnson compressor, built in England, 
there is a single poppet inlet valve of the gridiron type at each 
end of the cylinder. It has a large area, with a small lift, and is 
mounted in a peculiar way on the same spindle with the discharge 
valve (Figs. 53 and 54). Both valves are rendered easily accessible 



AIR INLET VALVES 



103 



by being placed in a chamber projecting horizontally from the end 
of the cylinder. This chamber is closed by a cast-iron plate held 
in place by a yoke and set-screw. The lift of the valve is con- 
trolled by an outside adjusting nut, c, on the spindle. The inlet 
valve is provided with a "lifter" (Fig. 53, d) by which it can be 
raised from its seat and thrown out of use, if it be desired tempora- 




Discharge 
Valve 



Inlet 
Valve 



Lifter 




SECTION 



Fig. 53. 



Figs. 53 and 54. — Johnson Air Valves. 




rily to make the compressor single-acting. The Johnson valve 
closes by gravity only, no springs being used. 

Humboldt Rubber Ring Valve. The older form of Humboldt 
wet compressor (see Fig. ^^) has a simple and ingenious valve 
(Fig. 55). It consists merely of a rubber ring of round cross- 
section which covers a series of horizontal slots, or ports, in a cyl- 
indrical casting set in the top of each air chamber. Three of these 
rings, a, with the slots, /, comprise the inlet valves in each end of 
the air cylinder; the casting, c, in which they are placed forming 
a part of the valve-chamber cover. The casting, c, is strengthened 



104 



COMPRESSED AIR PLANT FOR MIXES 



against the internal pressure by a series of webs, d. As the 
pressure in the air chamber falls during the inlet stroke the at- 
mospheric pressure expands the rubber rings, forcing them away 
from the slots, and allowing air to enter. Then on the reversal of 
the stroke the elasticity of the rings causes them to tighten up on 
their seats and close the ports. The valve openings are relatively 
large and permit free entrance of air. The discharge valve, b, has 
the same construction, but consists of a single ring only, of larger 
diameter and cross-section. These rubber valves are found to last 




Fig. 55. — Humboldt Rubber Ring Valve. 

well, as they are kept wet and are not exposed to any great degree 
of heat. They would be entirely unsuitable for dry compressors. 
Similar rubber valves are used in a wet compressor built by the 
Dingier Machine Works, Zweibruecken, Germany. The Gutter- 
muth valve is used in a later form of compressor built by the 
Humboldt Machine Works. It is a spring clack-valve, made of a 
rectangular plate of thin steel and provided with a grid seat. One 
side of the plate is coiled in a spiral, through the center of which 



AIR INLET VALVES 105 

passes a stationary rod or spindle, the inner edge of the spiral being 
inserted in a longitudinal groove in this spindle. By placing 
several valves side by side any desired area of opening can be 
furnished. To avoid the harmful effects of inertia, the valves 
are made of extremely thin plate, with delicately adjusted and 
sensitive springs, and by so arranging them that the current of 
air in passing through the valve into the cylinder undergoes but 
slight changes of direction, any serious eddying of the air around 
the edges of the plate is prevented. 

Leyner Flat Annular Valve. This recent form of valve, to- 
gether with its arrangement on the cylinder heads, is shown by Figs. 
56 and 57. Fig. 56 comprises a longitudinal section through the 
adjacent ends of the low- and high-pressure cylinders of a straight- 
line, two-stage compressor, indicating incidentally the circulation 
of the air through the intercooling tubes of both cylinders, as de- 
scribed in Chapter VI. At each end of the cut, left and right, is 
an outline cross-section, respectively of the low-pressure and 
high-pressure cylinder heads, showing the groups of intercooler 
tubes, with the valves themselves and their ports. 

The inlet and discharge valves being similar in form, a descrip- 
tion of the inlet only will be given (Fig. 57). It consists of a thin 
steel plate cut in a peculiar form. The outer, or seating portion, 
is a narrow annulus, with two slender internal arc-shaped strips 
terminating in a central ring, which is locked against the cylinder 
head by a steel nut encircling the piston-rod, thus holding the 
valve in place. The arc-shaped strips, connecting the seating part 
of the valve with the fixed central ring, are sufficiently long and 
flexible to serve as springs, and to permit the valve to open and 
close freely under very small differences of pressure. There is 
but one inlet and one delivery valve at each end of the cylinder. 
The inlet ports, D, D, four in number in each cylinder head, are 
curved, slot-like openings, arranged in the form of a circle. There 
are six similar but smaller discharge ports, E, E. Total area of 
inlet ports is about fourteen per cent., and of discharge ports, 
nearly nine per cent, of the piston area. The discharge valves are 
held in position by the hollow conical casting surrounding the 



io6 



COMPRESSED AIR PLANT FOR MIXES 




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AIR INLET VALVES 



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piston-rod stuffing-box. Their height of lift is limited by the 
stops, shown near F, F. 

This valve is simple in design, without separate springs, and con- 
sists of one part only. It cannot be doubted that the resistance 
to opening of the inlet valves is extremely small.* As the clearance 




Fig. 57. — Leyner Annular Inlet- Valve 

volume in these compressors is small (1.02 percent, of the cylinder 
volume in the 14-inch high-pressure cylinder mentioned in the 
foot-note), a high volumetric efficiency is stated to have been ob- 
tained,' a number of tests showing it to range from 94.6 to 97 per 
cent. 

* In a communication to the author the makers state that repeated tests of a 
14 and 22 X 26 inch, 2-stage compressor show a loss of intake pressure of only 
0.9 ounce. On a card made with a 20-scale spring, this would be represented by a 
difference of the inappreciable amount of 0.003 inch, between the intake and at- 
mospheric lines. The frictional loss through the delivery ports of the same com- 
pressor is 3 ounces. 



I0 8 COMPRESSED AIR PLANT EOR MIXES 

Arrangements for Admitting Inlet Air to the Compressor. It is 
of great importance that the intake air shall be as cool as possible. 
The colder the air the smaller is the volume occupied by a given 
weight of air taken into the compressor cylinder, and the greater the 
output. Taking in warm air involves loss of capacity and of econ- 
omv in production. Air. Frank Richards points this out in a con- 
vincing and simple way* "The volume of air at common tem- 
peratures varies directly as the absolute temperature. With the 
air supply at 6o° its absolute temperature is 52 1°, and its volume will 
increase or decrease T ^ for each degree of rise or fall of tempera- 
ture. Therefore, if in securing the supply of air we can get a dif- 
ference in our favor of 5 . . . we accomplish a saving of about 
one per cent. If a difference of temperature of io° can be secured 
two per cent, is saved, " practically without cost. The practice of 
taking air from the engine-room is a common one at mines, and is 
bad not only because such air is usually heated to a considerable 
degree, but is apt also to be charged with dust which causes un- 
necessary wear of valves and piston. 

Some means should be provided to convey to the compressor 
fresh air, taken preferably from some point outside of the building. 
A box or pipe of wood is better than one of iron, because of the 
smaller conductivity of wood. Its cross-section should be suf- 
ficient, say, at least one-half the area of the cylinder, to avoid loss 
from friction. To make such a connection conveniently the inlet 
valves should be enclosed in an external air chest on each end of 
the cylinder. Compressors having a single inlet valve, such as 
the Xorwalk, In gersoll- Sergeant, Sturgeon, etc., are better adapted 
than some of the others for making this arrangement. In any 
case, care should be taken to prevent the entrance of dust, leaves, or 
rubbish. If the inlet be left open, particles floating in the air may 
be drawn in by the strong current, and obstruct the valves or in- 
jure their seats and the smooth working surfaces of piston and 
cylinder. In such a design as that of the Ingersoll-Sergeant piston 
inlet, it is essential that the outer end of the hollow rod be covered, 
because in case of derangement the valves are not so accessible as 

* ''Compressed Air," p. 55. 



AIR INLET VALVES 109 

ordinary poppets. This protection is provided in recent designs 
of this compressor. By building a suitable conduit from the out- 
side of the compressor house to the air box enclosing the inlet valves, 
it is obvious that a greater saving can be effected in winter than in 
summer, but even in warm weather some advantage is gained, 
especially if the conduit opens on the north side of the building, out 
of reach of the sun's direct rays, and is carried vertically to some 
height above the ground level. 



CHAPTER VIII 
DISCHARGE OR DELIVERY VALVES 

The conditions affecting the action of the discharge valves of a 
compressor are wholly different from those which govern the suction 
or inlet valves. While the latter must be capable of opening 
under very small differences of pressure, the discharge valves are 
subjected to a heavy pressure on both sides. Furthermore, owing 
to unavoidable irregularities in the use of the air, the receiver 
pressure usually fluctuates considerably, so that the point of the 
stroke at which the discharge valves open cannot depend solely on 
the conditions, as to the ratio of compression, etc., under which 
the compressor itself is working. The time of opening must de- 
pend also on the relation between the variable pressures in 
cylinder and receiver. 

For this reason, the sensitiveness of operation essential in inlet 
valves is unnecessary for the discharge valves. The chief require- 
ments are that they shall be free to open when the cylinder pressure 
exceeds that of the receiver, shall fit accurately on their seats, and 
close promptly at the end of the stroke. Delay in closing, or leak- 
age between valve and seat, are far more serious than in the case of 
inlet valves, because these defects are equivalent to an increase of 
the piston clearance and consequent reduction of the volumetric 
capacity of the cylinder. The leakage of even a small quantity of 
compressed air back into the cylinder is equivalent to the loss 
caused by an abnormally large clearance space. The conditions 
under which discharge valves operate, therefore, are such as to 
afford a relatively limited field for innovation or improvement, 
as compared with inlet valves. 

Poppet Discharge Valves. Aside from a few designs in which 

no 



DISCHARGE OR DELIVERY VALVES 



III 



mechanical control in some form is introduced (see Chapter IX), 
nearly all discharge valves are of the poppet type. They are made 
heavier than inlet valves, with stronger springs to reduce hammer- 
ing on their seats. Though varying in details of construction, they 




Fig. 58. — Laidlaw-Dunn-Gordon Poppet Discharge Valve. 

may be represented fairly by the accompanying figures. Several 
other designs are also shown in the various sections of air cylinders 
illustrated in the preceding pages. Two of the ordinary forms of 
cup-shaped poppet, with internal springs, are shown in Figs. 58 
and 59. Occasionally they are of the mushroom type, somewhat 



112 



COMPRESSED AIR PLANT FOR MINES 



similar in shape to the inlet valve (Fig. 47), the spring then encir- 
cling the spindle. The valve may be of steel or bronze, with a 
bronze seat. To make it easier to keep them tight, the seating 
surfaces are usually coned. A group of several poppet valves are 
commonly employed, in order to avoid making them of large size 
and weight. The inertia of heavy valves causes destructive wear, 
under their high working pressure. Each valve should be readily 
accessible for adjustment, re-grinding, or renewal. They are there- 




Fig. 59. — Norwalk Poppet Discharge Valve. 



fore covered by caps screwed into the outer cylinder head; or, in 
some makes, by plates bolted on over the valve chamber. 

Cataract-Controlled Poppets. In another type of poppet dis- 
charge, the valve is not only provided with a spring, but its action 
is further modified by attaching the valve stem to the piston of a 
small cataract cylinder, containing either air or oil. This is to 
ease their movements and avoid hurtful shocks.* Oil-cataract 

* Similarly controlled poppets are also employed as inlet valves by some Euro- 
pean compressor-builders. 



DISCHARGE OR DELIVERY VALVES 



IJ 3 



valves are used, for example, in the compressors built by Schuech- 
termann and Kremer, Dortmund, Germany ; * air-cataracts in 
those of R. Meyer, Muhlheim-Ruhr ; G. A. Schuetz, Wurzen; 
Menck and Hambrock, Altona, and the Humboldt Machine Works, 
Kalk. (The rubber ring discharge valve, of the last-named build- 
ers, has already been referred to, in connection with Fig. 55.) 

These valves are employed to a considerable extent in Europe, 
but are not well known in this country. Some of them are 




Fig. 60. — "Express" Poppet Valve, Riedler Compressor. 

very satisfactory, provided the piston speed be slow; for high- 
speed compressors they do not work with sufficient promptness 
to prevent " slip " or leakage of some of the compressed air back 
into the cylinder. The chief object sought' in these cataract move- 
ments is attained in another way — by the partial control of an ac- 
companying Corliss valve — in the " Cincinnati " valve gear of the 
Laidlaw-Dunn- Gordon Co., described in Chapter IX (see Fig. 64). 
Riedler Discharge Valve. A poppet discharge valve entirely 
different in design is shown in Fig. 60, representing one of several 
patterns employed in the Riedler compressors. This is a light, 

* Described in London Engineering, Dec. 12th, 1902. 



114 COMPRESSED AIR PLANT FOR MINES 

cylindrical valve, A, provided with packing rings D. The cylinder 
in this case is vertical, and the piston, L, carries at its periphery the 
plate P, held in place by the stud N and the spring M. When 
closed the valve seats on the plate E, being held against it by the 
air pressure in the discharge passage acting on the under side of the 
upper flared end. In this position the round air ports near the 
lower edge of the valve are closed by the valve guide, at C,C. As 
the piston advances, and when the cylinder pressure exceeds that in 
the receiver, the valve is opened by the air pressure on the upper 
side of the flared end. This movement of the valve is cushioned 
by the air trapped above the guide, B, B. On reaching the end of 
the stroke, the plate P, on the piston, strikes the lower edge of the 
valve and closes it against its seat E, the shock being cushioned by 
the springs F and M. The double cushioning, in both opening 
and closing, tends to durability; and, moreover, it should be re- 
membered that, when the plate P strikes the valve, the crank is 
nearly on its center, so that the piston is moving very slowly. The 
standard mechanically controlled air-valve motion of the Riedler 
design is described in Chapter IX. 

Several other forms of discharge valve will be noted later, in 
connection with mechanically controlled valve motions. 

Discharge Area for Air Cylinders. The volume of air to be 
discharged from the cylinder having been reduced by compression 
to a small fraction of the volume occupied at atmospheric pressure, 
it might appear that the total area of the discharge valves could be 
made much smaller than the inlet area, without producing ex- 
cessive frictional resistance. But the compressed air must be 
forced out of the cylinder in a relatively short period of time. While 
the air enters throughout nearly the entire stroke, the delivery must 
take place while the piston is making, say, the last third or quarter 
of the stroke. Therefore, in a compressor of ordinary design, with 
several poppet inlet and discharge valves, the total discharge area 
should be about equal to the inlet area, provided the piston speed 
be moderate. When the inlet area is concentrated in a single 
valve (for example, like that of the Ingersoll-Sergeant piston inlet), 
the discharge area is made about double the inlet area, though this 



DISCHARGE OR DELIVERY VALVES 115 

relation varies in cylinders of different sizes, being proportionately 
greater in the larger compressors. Obviously, other things being 
equal, the discharge area should increase with the piston speed. 
For a speed of 300 feet per minute, the best results are obtained 
by making the discharge area, say, 10 per cent, of the cylinder area ; 
for speeds of 450 to 500 feet per minute, 15 per cent.* In some 
compressors, however, the discharge area is as small as from 8.5 
to 9.5 per cent. 

The above considerations apply in a measure also to the passages 
through which the air passes from the discharge valves to the pipe 
leading to the receiver. In some designs these are too restricted 
to permit a free flow of the air. The velocity of discharge should 
be made as small as possible, to minimize the resistance due to 
friction ; otherwise, during the period of delivery the pressure of 
the compressed air in the cylinder will rise momentarily above the 
normal, and then drop back after the air has passed out to the 
receiver. This causes a loss of power and unnecessary strains on 
the moving parts of the compressor. The amount of loss from 
this cause is represented by the irregular area of the air card which 
lies above a horizontal line drawn through the point corresponding 
to the pressure at the end of delivery. When the discharge A^alves 
first open, the piston is moving at a high velocity, and equilibrium 
with the receiver pressure is only attained as this velocity decreases 
toward the end of the stroke. 

* W. L. Saunders, Compressed Air, Dec, 1896, p. 153. 



CHAPTER IX 

MECHANICALLY COXTROLLED VALVES AXD VALVE 

MOTIONS 

The disadvantageous features of inlet valves whose opening 
and closure depend primarily upon difference of air pressure have 
led to the introduction of numerous mechanically controlled 
valves. By their use fewer valves are required, as a rule, because 
they may be made much larger and have a higher lift. As dis- 
tinguished from ordinan* poppet valves, they are operated or con- 
trolled by being in some way connected with the rotary or recipro- 
cating parts of the compressor. A prompter opening is thus 
secured, so that the compressor is enabled to take more nearly a 
full cylinder of air at each stroke. 

In some designs the connection between the valves and their 
operating mechanism is absolutely positive and fixed for any one 
setting of the valves., which are timed with respect to the piston 
stroke, so as to open at the instant the clearance air has been re- 
expanded to atmospheric pressure,, and to close at the end of the 
stroke. Other designs involve the use of springs, which modify 
to some extent the operation of the controlling mechanism, thus 
allowing for variations in working conditions, as well as for in- 
accuracies of adjustment or slight derangements caused by wear 
of parts. Still other valve motions exert a partial control, which, 
within narrow limits, leaves the valve free to act under difference of 
air pressure inside and outside of the air cylinder. 

As a rule, in the recent designs of mechanical valve motions the 
inlet valves only are positively controlled, and in most cases the 
type of valve used is a modified form of the Corliss. But while 
mechanically controlled valves are often employed for the low- 

116 



MECHANICALLY CONTROLLED VALVES AND VALVE MOTIONS II 7 

pressure cylinders of stage compressors, they are not suitable for the 
high-pressure cylinders; the inlets of these are subjected to heavy 
pressures on both sides, and are best allowed to open and close 
solely under the difference between these pressures, which is more 
than sufficient to produce prompt action of the valve at the proper 
time. Poppet valves are therefore generally used for this service. 

Mechanical Control for Discharge Valves. The adoption of any 
system of mechanical control for discharge valves is a matter of some 
difficulty, because of the fluctuations of receiver pressure under 
which these valves are compelled to operate. In attempting to 
open them by a positive mechanical movement, at a fixed point 
of the stroke, two cases may occur: i, in event of a drop in receiver 
pressure below the normal, the valves and their controlling mech- 
anism would be subjected to a heavy strain, before the point of 
opening is reached, due to the excess of cylinder pressure; and, 
2, if the pressure in the receiver should rise above the normal, 
the valves, until permitted to open by the controlling gear, would 
be held forcibly on their seats, against the excess of receiver press- 
ure. In either case, derangement or breakage of the valves or of 
some part of the controlling mechanism may occur. 

Hence, in order that the discharge valves may adjust themselves 
automatically to the varying conditions, some degree of freedom 
as to their time of opening must be allowed. It is true that the 
range of fluctuation in receiver pressure is lessened by the use of 
air-pressure regulators (Chapter XI) ; nevertheless, only a partial 
mechanical control of these valves is practicable for any service in 
which the consumption of air is variable or intermittent. More- 
over, Corliss valves of the ordinary patterns used for compressors 
do not serve well for discharge valves where the ratio of compres- 
sion is greater than, say, three or three and one-half ; because they 
must then be set to open too late in the stroke to permit a free 
discharge. This applies to single-stage compressors, as designed 
for ordinary service, as well as to the high-pressure cylinders of two- 
stage machines. A number of devices have been introduced for 
dealing with these conditions; such as the use of relief valves 
working in conjunction with mechanically operated discharge 



Il8 COMPRESSED AIR PLANT FOR MINES 

valves; or, as in one form of the Riedler compressor, the opening 
of the valve is governed in part by the air pressure, a very small 
free lift being allowed by the controlling mechanism for affording 
the necessary relief. 

Valve Motion of Norwalk Compressor. An adaptation of the 
Corliss valve gear has been used for many years for the low-press- 
ure cylinder of this compressor (Fig. 61). One large inlet and one 
discharge valve are set in chests at each end of the cylinder. Pop- 
pet valves are employed for the high-pressure cylinder. These are 
shown in Fig. 7, together with the cross-sections of the low-pressure 
Corliss valves in their respective chests. The main valve-rod, a, 
is driven by a drag- or return-crank, b, mounted on the crank-pin 
of the fly-wheel. The rod is pin-connected to a short lever, c, on 
the spindle of the forward inlet valve, and from this lever a link, d> 
passes to a corresponding connection with the inlet valve at the 
other end of the cylinder, the parts being so adjusted that one 
valve opens as the other closes. A positive movement of the valves 
is thus obtained. 

An essential feature of this valve motion is the introduction of 
the cams / / and g g, for operating the discharge valves. These 
cams are mounted in pairs on the respective inlet and discharge 
valve spindles, and form part of the short levers c. As each inlet 
valve oscillates, its cam rolls smoothly upon that of the discharge 
valve above it, the shape of each pair of cams being such that the 
discharge valve is opened full at the proper point of the stroke, i.e., 
when the pressure within the cylinder becomes equal to that in the 
discharge passage outside. Then, at the end of the stroke, when 
the cams move in the opposite direction, and while still rolling upon 
each other, the discharge valve is closed without shock by the con- 
necting link, e. This link is elastic, being made of two telescoping 
parts, somewhat on the principle of a dash-pot, thus allowing the 
freedom of movement necessary for dealing with variable receiver 
pressure. 

In recent years, a number of other compressor-builders have 
adopted modifications of the Corliss valve gear for the air cylinders. 

Nordberg Valve Motion. For single-stage compressors of this 



MECHANICALLY CONTROLLED VALVES AND VALVE MOTIONS 119 




z 
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120 



COMPRESSED AIR PLANT FOR MINES 



make, the inlet valves are of the Corliss type, poppets being used 
for discharge (see Fig. 35). The inlet valves are operated posi- 
tively from a triple wrist-arm, on the side of the air cylinder. This 
wrist-arm is driven by an eccentric on the crank-shaft, the con- 
necting rod being supported by an intermediate carrier arm, sus- 
pended from the engine frame. Connecting links pass from the 
wrist-arm to the valve levers. The lap of the valves can be altered, 
when necessary, by slightly shifting the angular position of the 
lever with respect to the valve spindle on which it is mounted. 
This is done by means of a pair of adjusting screws on the hub of 




Fig. 62. — Air Cylinder of Nordberg Compressor. 

the valve lever, the correctness of the valve motion being unaffected. 
The discharge valves are of the cup-poppet type. 

The double- and triple-stage compressors are provided with 
similar inlet valves, a modified Corliss valve, with double ports, 
being used for discharge (Fig. 62). This valve is shown open on 
the right- and closed on the left-hand end of cylinder. An opening 
in the center of the valve allows the air to discharge on both sides. 
In the axis of each Corliss valve are set a series of spring poppets, 
which act as relief valves when the receiver pressure falls below the 
normal. It will be seen, by reference to the above figures, that the 



MECHANICALLY CONTROLLED VALVES AND VALVE MOTIONS 121 

water-jacketed area on the cylinders is unusually large, jackets 
being applied wherever possible on the heads and around the 
valves, as well as on the cylinder barrels. 

Another form of Nordberg valve gear is used in compressors in- 
tended to be driven at constant speed — by belting or gearing from 
an electric motor, water-wheel, or engine used for other service. 
While the general construction of the air cylinders is the same as 




Suction 
Fig. 63. — Section of Air Cylinder. Laidlaw-Dunn-Gordon Co. 



shown in Fig. 35, the inlet valves are provided with a releasing 
mechanism. The valves are opened and closed as usual by wrist- 
plate links, when the air pressure is normal. But when the press- 
ure increases the inlet valves are released from the valve motion 
and held open until the pressure drops ; that is, the compressor is 
unloaded for the time being, useful work ceasing. The release is 
effected by introducing knock-off cams, similar to those used for 



122 



COMPRESSED AIR PLANT FOR MINES 



Corliss steam valves, these cams being operated by a loaded plunger 
to which the compressed air is admitted when the pressure exceeds 
the normal. With this gear the compressor is self-regulating to 
within small limits. For duplex compressors, added delicacy of 
regulation is obtained by designing the knock-off cams to unload in 
four successive steps, according to the variation in air pressure. 

Laidlaw-Dunn-Gordon Valve Motions. Several forms of me- 
chanical valve motions are made by these builders. One of them 
is shown in the arrangement of its valves, by Fig. 63, the inlet valves 




Fig. 64. — "Cincinnati" Valve Gear, Laidlaw-Dunn-Gordon Compressor. 

being of the usual Corliss pattern, with spring poppets for the 
discharge. 

Another design, recently brought out, is the " Cincinnati'' air 
valve gear (Fig. 64. See Fig. 19 also, for a plan and longitudinal 
section of the complete compressor). This valve motion is peculiar 
in the fact that a single Corliss valve, at each end of the cylinder, 
serves as both inlet and discharge. The cross-section of the valve, 
therefore, differs materially from the usual form, as shown by the 



MECHANICALLY CONTROLLED VALVES AND VALVE MOTIONS I 23 

cut. The valve at the right-hand end of the cylinder is in position 
for admitting inlet air, the air passage being indicated by dotted 
lines; while that at the left is open for discharge, the corresponding 
inlet being closed. A large poppet is set vertically just above the 
Corliss valve. The latter is timed to open the port sufficiently early 
in the stroke to leave the poppet free to rise whenever the pressure 
in the cylinder exceeds receiver pressure. At the end of the stroke 
the Corliss valve takes its inlet position (right-hand of cut), and at 
the same time, by shutting off the discharge, confines a small 
quantity of compressed air in the passage under the poppet. This 
air acts as a cushion, and allows the poppet to seat itself slowly and 
without shock, during the return stroke of the piston. The neces- 
sity for the usual sharp closure of the discharge is thus avoided; 
the spring may be made lighter, and the wear of both valve and 
seat is reduced. 

It will be seen that the fixed mechanical control of the valves 
is exerted at three points : opening and closing of the inlet, and 
closing of the discharge. In permitting the poppet to open freely 
by the combined action of both valves, one of the chief difficulties 
of applying mechanical control to discharge valves is eliminated, 
viz. : that of dealing with the variable receiver air pressure. This 
valve motion is well suited for running at high piston .speeds, as, 
for example, in the case of compressors driven by direct-connected 
motors. 

Allis-Chalmers Valve Motions. These are of several types, 
resembling in part some of those already described, but differing 
in many details. Fig. 65 shows a standard form for the duplex 
compressor, in which the Corliss inlet valves are operated from a 
triple wrist-arm, driven by an eccentric on the fly-wheel shaft. 
The discharge valves (five in number for ordinary sizes of com- 
pressor) are spring-poppets of the cup form. 

Another design of discharge valve employed by these builders 
consists of a light cup-shaped poppet, without a spring, which is 
permitted to open freely, according to the air pressures, but is 
closed positively by a plunger actuated from a separate wrist-plate 
and eccentric. A single valve is placed in each cylinder head. 



124 



COMPRESSED AIR PLANT FOR MINES 



The plunger, carried by exterior guides, works within the valve and 
is so timed that it forces the valve to its seat just at the end of the 
stroke. On the return stroke of the piston the plunger recedes, 
while the valve is held on its seat by the receiver pressure until the 
pressure within the cylinder rises sufficiently to open it. In closing 
the valve, the advancing plunger is cushioned on the air in the cup 
of the valve, so that the latter is seated without shock. 

Still another form of Allis- Chalmers valve-gear consists of me- 
chanically operated Corliss valves for both inlet and discharge. 



-ygPWB'B 

Irani 










«3 


^^TsS^H 


J 






'A 
i 





Fig. 6v — Standard Air Valve Motion. Allis-Chalmers Co. 



The time of closing of the discharge valve is adjusted for the 
maximum working pressure. To allow for variations, small 
auxiliary spring poppets are provided, to act as relief valves. 
These open freely when the receiver pressure falls below the 
working pressure for which the positively operated Corliss valves 
are set. 

Sullivan Valve-Motions. In the cross-compound, two-stage 
compressors of this make, Corliss valves are employed for the in- 
take of both low- and high-pressure cylinders. Fig. 66 is a 
longitudinal section of the high-pressure cylinder, the discharge 



MECHANICALLY CONTROLLED VALVES AND VALVE MOTIONS 1 25 

valves of which are of the poppet form. Corliss discharge valves 
are used in the intake cylinder, but are accompanied by poppet 
relief valves, similar in principle to those in one of the Allis-Chal- 
mers designs described above. The air valve gear is driven by the 
usual eccentric and wrist-plate motion. 

Both air cylinders of the Sullivan tw T o-stage, straight-line com- 
pressor are fitted with Corliss inlet valves, operated from an eccen- 
tric pin attached to the main crank-pin. The discharge valves in 




Fig. 66. — Sullivan Air Cylinder, showing Corliss Inlet Valves. 



this compressor are arranged in a rather unusual manner, being 
placed in the lower, instead of the upper, part of the cylinder 
heads. In some of the other patterns of these makers, the poppet 
discharge valves are set radially around the upper part of the cylin- 
der-head castings. (See, for example, the cross-section of two- 
stage compressor, Fig. 35.) By this arrangement the piston clear- 
ance can be made very small, and the valves, placed in removable 
seats, are surrounded by the water-jackets. 



126 COMPRESSED AIR PLANT FOR MINES 

Other Mechanically Controlled Valve Motions, resembling in 
principle those already noted, though differing in details of con- 
struction, are employed in a number of compressors which need 
not be described here, such as the Franklin, Clayton, Rix, American, 
etc. With but one exception, the mechanically controlled air 
valves referred to in the preceding pages are modifications of the 
Corliss rotary or oscillating valve. A wholly different type, how- 
ever, is found in the 

Riedler Air- Valve Motion. This ingenious valve motion has 
undergone several radical modifications since it was introduced, 
about twenty years ago. Its present design, as built until recently 
by the Allis- Chalmers Co., is illustrated in Figs. 67, 68, and 69. 
Fig. 67 comprises side and half-end elevations of the low-pressure 
air cylinder. The mechanical control is exerted through a wrist- 
plate, A, supported on the side of the cylinder and operated through 
a lever from an eccentric on the fly-wheel shaft. Back of the wrist- 
plate is a horizontal sliding plate, B, to which the links, E, E, are 
pinned. This plate is caused to reciprocate, through a distance 
equal to the permitted lift of the valves, by two cams cut on the 
periphery of the wrist-plate and working against studs set in B. 
The motion thus transmitted through the links, E, oscillates the 
transverse rock-shafts, D, and produces the necessary throw of the 
forked levers, C, which control the closure of the valves. The 
rock-shafts are carried in bearings outside of the cylinder-head 
housings. 

The four valves, two suction and two delivery, are almost iden- 
tical in design, consisting of an annular seating portion, connected 
by radial ribs to the central disks. They are of a double-seated 
poppet type, the air passing within the seating ring as well as 
around its periphery. Screwed into the valve is a long stem, 
passing out through a sturhng-box in the cylinder head and into a 
bonnet bolted on outside. Within the bonnet is a dash-pot whose 
piston is attached to the valve stem. 

The operation of the inlet valve, F, Fig. 68, is as follows: At 
the beginning of the stroke the forked lever, C, is depressed by the 
rock-shaft and link, noted above. This leaves the valve free to 



MECHANICALLY CONTROLLED VALVES AND VALVE MOTIONS 1 27 




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COMPRESSED AIR PLANT EOR MINES 




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MECHANICALLY CONTROLLED VALVES AND VALVE MOTIONS 1 29 




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COMPRESSED AIR PLANT FOR MINES 



open, its movement being steadied by the dash-pot piston, G. 
The resistance presented by this piston is regulated by the adjusting 
screw, H. For ordinary sizes of compressor the total lift of the 
valve is one inch, giving a large area of opening. (The ioj-in. 
valve shown in the cut, which is for the low-pressure cylinder of a 
24" and 38" X 48" compressor, has an area of 45 sq. ins.) 
Toward the end of the stroke the forked lever begins to rise, there- 
by bringing the valve gradually nearer its seat, as the piston velocity 
decreases. In completing its movement the lever forces the valve 
upon its seat promptly at the end of the stroke. By this device, the 
valve attains its maximum lift and area of opening toward the 
middle of the stroke, when the velocity of the inflowing air is great- 
est, and is brought nearer its seat as the flow diminishes, so that the 
complete closure is effected instantaneously at the proper time. 

A similar control is exerted over the delivery valve, though the 
details of its bonnet, dash-pot, and forked lever are quite different, 
as shown by Fig. 69.* At the proper point of the stroke the 

lever is depressed, so that the 
valve is free to open when the 
air pressure in front of the 
advancing piston has reached 
receiver pressure. Then, as 
the velocity of outflow dimin- 
ishes toward the end of the 
stroke, the valve is forced 
nearer its seat, and a prompt 
closure takes place the instant 
the stroke reverses. As a re- 



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Cylinder Head 



Fig. 70. — Cam-Controlled Inlet Valve. 



suit of this mechanical control, together with the action of the 
dash-pot, the operation of the Riedler valves is attended with but 
little shock, thus permitting a high piston speed. 

Cam-Controlled Inlet Valve. At the Lens colliery, in France, 
a cam movement has been successfully applied for controlling the 
opening of a poppet inlet valve (Fig. 70). The stem of the valve 

* The delivery valve in the cut is the same size as the suction valve previously 
described. It is designed for a smaller compressor, 15" and 24" X 36". 



MECHANICALLY CONTROLLED VALVES AND VALVE MOTIONS 131 



is provided with a spiral spring, and projects from the cylinder 
head, as usual. At the beginning of the stroke the valve is opened 
rapidly by a cam, a, of peculiar shape, playing against the end of the 
stem. The cam is mounted upon a small shaft which is geared to 
revolve once for each revolution of the compressor. At the end of 
the stroke the cam allows the valve to close under the action of the 
spring.* 

Sturgeon Inlet Valve. This peculiar valve, of an air com- 
pressor made in England, furnishes an example of a positive 
movement entirely different in principle from those already de- 
scribed. It is a large annular valve, c (Fig. 71), encircling the piston 
rod in each cylinder head, and 
is operated directly by the 
movements of the rod it- 
self. I The connection be- 
tween the valve and rod is 
frictional only, being brought 
about by a gland, e, which 
serves also to form a stumng- 
box for the piston. By tight- 
ening or loosening the nuts, 
a, of the bolts by which the 
gland is attached to the valve 
flange, any desired amount of 
grip upon the piston rod can 
be obtained. This frictional 
grip is regulated so that the valve will not be opened until the 
clearance air has been re-expanded nearly to atmospheric pressure. 
Flanges on the ends of the valve limit its play in each direction, 
controlling the amount of lift and area of opening. The valve 
and stuffing-box together form the bearing of the piston rod in 
each cylinder head. At the end of the stroke a recess in the 
piston receives the large inner flange of the valve so as to 
diminish the clearance. The mechanism is simple and its work 
satisfactory. 

* H. W. Hughes, "Text-book of Coal Mining," p. 55. f Idem, p. 53. 




Fig. 71. — Sturgeon Inlet Valve. 



132 COMPRESSED AIR PLANT FOR MINES 

An arrangement resembling the above has been used in a 
compressor made by the Dover Iron Co., Dover, N. J. 

The well-designed Koster valve is of the piston type, almost 
unknown in this country for air-compressor service. It is now em- 
ployed by several European makers, among them: Pokorny & 
Wittekind, Frankfort, Neumann & Esser, Aachen, and W. H. 
Bailey & Co., Manchester, England. The valves, both inlet and 
discharge, are very large in area and are mounted on a longitudinal 
spindle deriving its reciprocating motion from an eccentric on the 
crank-shaft. Positive opening and closure are imparted to the 
inlet valves, but the opening of the delivery port is effected by an 
independent poppet, encircling the spindle and provided with a 
light spring. This valve motion constitutes a highly developed 
type, and is both reliable and efficient. 



CHAPTER X 

PERFORMANCE OF AIR COMPRESSORS 

The performance or duty of air compressors may be designated 
in several different ways. 

First. A standard of rating, useful for ordinary purposes, 
is the duty in terms of cubic feet of free air compressed per 
minute to a given pressure. The theoretical output is found 
by multiplying the net piston area in square feet by the dis- 
tance travelled by the piston in feet per minute. The actual out- 
put will be less than the theoretical on account of various losses due 
to leaks, clearance, induction of warm air, friction of inlet valves, 
etc. In a properly designed compressor an allowance of fifteen per 
cent, to eighteen per cent, is sufficient to cover these losses, which 
must not be confounded with the mechanical loss of work — that is, 
the work expended in overcoming the friction of the compressor — 
and the loss of work due to the heating of the air under compression. 

Having found the capacity of the compressor, in terms of cubic 
feet of free air, the volume V, occupied by this air at any given 

VP 

pressure, P', is calculated by the formula already given: V = — , 

in which the following values are now assigned, viz. : 

V = initial volume of given quantity of air. 

P = normal absolute pressure of atmosphere (14.7 lbs.). 

P' = absolute pressure of air under compression, i.e., gauge 
pressure + 14.7 lbs. 

For example, 100 cu. ft. of free air, compressed isothermally to 
65 lbs. gauge pressure, will occupy a volume: 



X L. IOO X 14.7 _ - 

V = — = 18.45 cu. ft. 

65 + 14.7 ^ 



134 COMPRESSED AIR PLANT FOR MINES 

Conversely, the volume of free air represented by 18.45 cu - ft- 
of air at 65 lbs. gauge pressure is : 

v V'F 18.45 (65 + 14-7) ft 

\ = — =— = — = IOO CU. ft. 

p 14.7 

By applying the 15 to 18 percent, allowance for losses stated 
above, this allowance depending on the type of compressor, re- 
sults are obtained sufficiently accurate for practical purposes. 
As the volumetric output of a compressor of given size of cylinder 
depends on the density of the intake air, it will obviously be 
reduced when working at an altitude above sea-level. (See Chap- 
ter XIII.) 

Second. The size of the compressor may be designated in 
terms of the horse-power developed by the steam end, indicator 
cards being taken while running at normal working speed and while 
the usual volume of air is being consumed. 

Third. The effective horse-power of the quantity of com- 
pressed air delivered is determined from an indicator card, taken 
from the air cylinder. In testing a compressor it is customary 
to take a series of cards, simultaneously from both ends of the 
steam and air cylinders. They may then be compared, as shown 
by Fig. 20. 

If indicator cards be not available, the theoretical horse-power 
for single-stage adiabatic compression may be calculated by the 
formula : 



H.P. = 



144 PVw - (Y 



J, in 



which 



33,ooo(n— 1) LV P 

P = normal atmospheric pressure per square inch (14.7 pounds). 

P' = final absolute pressure per square inch. 

V = the volume of free air compressed per minute, in cubic feet. 

n = exponent of the compression curve, as given under the the- 
ory of air compression, viz.: for adiabatic compression, n = 1.406, 
and varies down to 1.18 or 1.2, depending on the efficiency of the 
cooling arrangements. For the best single-stage compressors, n = 
say, 1.25 or 1.3. 

For isothermal compression, the expression for the horse-power is : 



PERFORMANCE OF AIR COMPRESSORS 135 

H.P. = -^- XPV( Nap. log. ~ ) 
33,000 v P / 

Table V shows the horse-powers required, under the conditions 
named, to compress one cubic foot of free air per minute to different 
gauge pressures, by single-stage compression : 

Table V 







Single-Stage Compression, from Atmospheric 






Pressure at Sea-Level. 


Initial Temp., 60° Fah. 






Horse-Power Required 


to Compress 


[ CU. FT. OF 






Free 


Air. 






Atmos- 










pheres Ab- 
solute, or 
Ratio of 








Gauge 








Pressure, 
Lbs. 


Theoretical Horse-Power. 


Actual Horse- Power (Approx.). 


Compres- 










sion. 






Allowance for 


Allowance for 






Isothermal 


Adiabatic 


Losses above 


Losses above 






Compression. 


Compression. 


Adiabatic Com- 
pression, 15$. 


Adiabatic Com- 
pression, 20$. 


20 


2.36 


-055I 


.0626 


.0720 


-075I 


25 


2.71 


.0637 


.0741 


.0852 


.0890 


3° 


3-°4 


■°7 1 3 


.0843 


.0970 


.1011 


35 


3-3* 


.0782 


.0941 


.1082 


.1129 


40 


3-7 2 


.0842 


.1029 


.1183 


-1234 


45 


4.06 


.0895 


.1115 


.1282 


-1338 


5° 


4.40 


.0950 


.1191 


.1370 


.1430 


55 


4-74 


.0994 


.1269 


.1460 


.1522 


60 


5.08 


.1041 


-1337 


-1537 


.1604 


65 


5-42 


.1081 


.1401 


.1610 


.1681 


70 


5-76 


.1123 


.1468 


.1690 


.1761 


75 


6.10 


.1162 


-1535 


-1765 


.1842 


80 


6-44 


-II95 


- I 59 I 


.1830 


.1910 


85 


6.78 


.1224 


.1651 


.1900 


.1961 


90 


7.12 


.1256 


-i7°3 


-1955 


.2040 


95 


7.46 


.1287 


.1760 


.2024 


.2112 


100 


7.80 


-1315 


.1807 


.2080 


.2168 


no 


8.48 


.1366 


.1894 


.2180 


.2272 


125 


9-50 


.1442 


.2025 


.2328 


.2430 



In columns three and four of Table V are shown the theoretical 
horse-powers required for isothermal and adiabatic compression. 
The results of isothermal compression are wholly unattainable in 
practice, and are placed here only for purposes of comparison. 
They represent an ideal which it is desirable always to keep in view. 

* The Naperian or hyperbolic logarithm of a number is obtained by multiplying 
the common logarithm by the constant 2.302585. 



136 COMPRESSED AIR PLANT FOR MINES 

The figures given in the column of adiabatic compression are based 
on the assumptions that there is no radiation of heat from the air 
cylinder, and that the temperature of the air after delivery has 
become normal, its volume being therefore reduced to that which 
is practically available for use. No allowances are included in 
these figures to cover losses other than that due to the heating of the 
air under compression. But the full amount of loss represented by 
adiabatic compression can never be suffered in the operation of com- 
pressors, however imperfect their design. The actual compression 
line must always be lower than the adiabatic line, because of the 
radiation of heat through the cylinder walls. In ordinary, single- 
stage compressors, properly water-jacketed and run at a reasonable 
piston speed, the compression line falls considerably below the 
adiabatic line. Whatever diminution of loss is effected by cooling 
of the air in the cylinder may therefore be credited against the other 
unavoidable losses, partially offsetting them, viz.: frictional or 
mechanical loss in the compressor, friction of inlet valves, heating 
of the intake air by contact with the hot metal surfaces, and piston 
clearance of the cylinder. These losses are variable in amount, 
depending on the design of the compressor. 

In the absence of indicator cards, giving the actual results in 
individual cases, estimates based on practice may be made of the net 
power loss experienced in operating compressors, which will be con- 
venient for reference. With this understanding, an attempt is 
made, in columns five and six of the above table, to show the actual 
horse-power required to compress one cubic foot of free air, under 
the conditions stated at top of the columns. Thus, in column five, 
fifteen per cent, is assumed as a fair estimate, in case of well- 
designed and operated single-stage compressors, of the additional 
power required, over and above that for theoretical adiabatic com- 
pression; this fifteen per cent, being taken as: the loss in purely 
adiabatic compression, minus the effect of ordinary water-jacket 
cooling, plus the other three losses mentioned at end of preceding 
paragraph. In column six, the power consumed in adiabatic 
compression is increased by twenty per cent., which represents 
relatively poorer work. 



PERFORMANCE OF AIR COMPRESSORS 137 

The figures in columns 3 and 4 or 5 and 6 (which are for 
jree air), if multiplied by the corresponding ratios of compression 
(column 2), will give the respective theoretical and actual power 
costs of furnishing one cubic foot of compressed air, at the gauge 
pressures stated. 

Work Done by Stage Compressors. The theoretical horse- 
power required to compress a given volume of free air to any given 
pressure, P', is computed for a two-stage compressor by the for- 
mula: 

2 X 144 PVw r / P' 



H.-P. = " X 



r[(f)--0 



33,000 n 

This formula is derived from that for single-stage compression 
by dividing equally between the cylinders the total work done, and 
then taking the sum of the two. 

For three-stage compression the formula becomes : 



^3,000 w-i L\P / J 



H.-P. = 

33,000 

Reducing the constants, and for a volume of one cubic foot of 
free air, these formulas may be simplified thus : 

r / p' \ 0144 -1 
Two-stage, H.-P. = 0.449 |_ v "p J ~ 1 J 

r~ / p'\ 0.00952 -1 
Three-stage, H.-P. = 0.6735 [_ (^5- J ~ 1 J 

In these expressions it is assumed that the work of compres- 
sion in each cylinder is done adiabatically, and that the temper- 
ature of the air after leaving the cylinder is reduced by intercool- 
ing to the initial temperature. 

For convenience, the horse-powers for stage compression at sea- 
level, both theoretical and actual, are given for a few gauge press- 
ures in the following table; the figures in the fifth and seventh 
columns being taken as an approximation to the results obtainable 
in practice from stage compressors of the usual designs. 

At elevations above sea-level, P is less than 14.7, and for any 
given altitude the atmospheric pressure must therefore be known. 



*38 



COMPRESSED AIR PLANT FOR MINES 

Table VI 





Ratio of Com- 


HORSE- 


Power per Cubic Foot of Free Air. 






Two-Stage 


Three-Stage 


Gauge 




Compression. 


Compression. 


Pressure, 


pression 
P' 

"P 








Lbs. 


Isothermal 
Compression. 




Actual H - 




Actual H.- 






Adiabatic 


P., on basis 


Adiabatic 


P., on basis 








Compres- 


of Adia. 


Compres- 


of Adia. 








sion. 


Comp'n 
+ 18*. 


sion. 


Comp'n 
+ 15*. 


70 


5-76 


0.1123 


0.129 


0.152 






80 


6.44 


-"95 


.138 


.163 






90 


7.12 


.1256 


.147 


-173 






100 


7.80 


-1315 


.154 


.182 


0.145 


0.167 


120 


9.16 


.1420 


.169 


.199 


-158 


.182 


140 


10.50 


.1508 


.181 


.213 


.169 


.194 


160 


11.88 


.1583 


.192 


.226 


.179 


.206 


180 


13.24 


.1654 


.202 


.238 


.188 


.216 


200 


14.60 


.1720 


.212 


-250 


.196 


.225 


250 


18.00 


-1853 


-232 


-274 


.213 


-245 


300 


21.40 


.1963 


.249 


-294 


.228 


.262 


350 


24.80 


.2058 


.264 


.312 


.241 


-277 


400 


28.20 


.2140 


-277 


-327 


-252 


.290 


45° 


31.62 


.2215 


.289 


•341 


.262 


.301 


500 


35- 01 


.2280 






.271 


-311 


55o 


38.41 


-2339 






.280 


.222 


600 


41.80 


•2393 






.288 


■33 1 


650 


45.21 


-2443 






-295 


-339 


700 


48.62 


.2490 






.301 


-346 


800 


55-42 


-2574 






-3 J 4 


.361 



Table VII will be found useful for making calculations in 
which are used volumes and mean cylinder pressures for isother- 
mal and adiabatic compression. 

In this table the mean pressures per stroke, given in the 
fifth and sixth columns, are obtained from the formulas for iso- 
thermal and adiabatic single-stage compression, which precede 
Table V, except that they are here expressed in terms of foot- 
pounds of work, instead of horse-power. These formulas may be 
put respectively in the following forms : p/ 

Mean pressure per stroke (isothermal) = P X Nap. log. -5- 

Mean pressure per stroke (adiabatic) = 3.463 P [ \~p ) ~ I \ 

1.406 



* The constant 3.463 = 



n— 1 



.406 



PERFORMANCE OF AIR COMPRESSORS 



1 39 



Table VII* 



u 

3 
en 
in 

V 

u 

Ph 
CD 


in 

cu 
u 
tU 

O, 
M 
O 

s 


olume with Air at 
Constant Tempera- 
ture. 


-4-> 

O 

£ 

"3 


!ean Pressure per 
Stroke; Air at Con- 
stant Temperature. 
Pounds. 


ean Pressure per 
Stroke; Air Not 
Cooled. Pounds. 


emperature of Air; 
Not - Cooled. De- 
grees Fahrenheit. 


O 


< 


> 


> 


S 


S 


H 





1 


1 


1 








6o° 


I 


1-068 


-93 6 3 


.9500 


.96 


-975 


7i 


2 


1. 136 


-8803 


.9100 


1.87 


1. 91 


80.4 


3 


1.204 


-8305 


.8760 


2.72 


2.80 


88.9 


4 


1.272 


.7861 


.8400 


3-53 


3-67 


98 


5 


1-340 


.7462 


.8100 


4-3° 


4-5° 


106 


IO 


1.680 


-5952 


.6900 


7.62 


8.27 


H5 


15 


2.020 


-495° 


.6060 


10.33 


11. 51 


178 


20 


2.360 


-4237 


-543° 


12.62 


14.40 


207 


25 


2.700 


•3703 


.4940 


14-59 


17.01 


234 


3° 


3.040 


-3289 


-4538 


16.34 


19.40 


252 


35 


3-381 


-2957 


.4200 


17.92 


21.60 


281 


40 


3.721 


.2687 


-393° 


19.32 


23.66 


3°2 


45 


4.061 


.2462 


-3700 


20.57 


25-59 


321 


5° 


4.401 


.2272 


-3500 


21.69 


27-39 


339 


55 


4.741 


.2109 


■33™ 


22.76 


29.11 


357 


60 


5.081 


.1968 


-3 J 44 


23-78 


30-75 


375 


65 


5-423 


.1844 


.3010 


24-75 


32-32 


389 


70 


5.762 


-1735 


.2880 


25.67 


33-83 


405 


75 


6.102 


.1639 


.2760 


26-55 


35-27 


420 


80 


6.442 


-1552 


.2670 


27-38 


36.64 


43 2 


85 


6.782 


.1474 


.2566 


28.16 


37-94 


447 


90 


7.122 


.1404 


.2480 


28.89 


39-iS 


459 


95 


7.462 


.1340 


.2400 


29-57 


40.40 


472 


100 


7.802 


.1281 


.2320 


30.21 


41.60 


485 


io 5 


8.142 


.1228 


.2254 


30.81 


42.78 


496 


no 


8.483 


.1178 


.2189 


31-39 


43-9i 


507 


ii5 


8.823 


■^33 


.2129 


3i-98 


44-98 


5i8 


120 


9.163 


.1091 


-2073 


32-54 


46.04 


529 


125 


9-5°3 


.1052 


.2020 


33-o7 


47.06 


54o 


130 


9-843 


-1015 


.1969 


33-57 


48.10 


55o 


135 


10.183 


.0981 


.1922 


34-05 


49.10 


560 


140 


10.523 


-0950 


.1878 


34-57 


50.02 


57o 


145 


10.864 


.0921 


-1837 


35-09 


51.00 


580 


150 


11.204 


.0892 


.1796 


35-48 


51.89 


589 


160 


11.880 


.0841 


.1722 


36.29 


53-65 


607 


170 


12.560 


.0796 


-1657 


37.20 


55-39 


624 


180 


13.240 


-0755 


-1595 


37-9° 


57-oi 


640 


190 


13.920 


.0718 


.1540 


38.68 


58.57 


657 


200 


14.600 


.0685 


.1490 


39-42 


60.14 


672 



* Kents' " Mechanical Engineers' Pocket Book." Taken from a table in Rich- 
ards' " Compressed Air," p. 20. 



I-P COMPRESSED AIR PLANT FOR MINES 

The work done during one stroke of the compressor is found 
by multiplying the mean pressure by the volume in cubic feet, 
V, traversed by the piston. 

When air is compressed adiabatically, the relation between the 
temperature T, of the air at the beginning of compression, and the 
temperature at the end, T\ is shown by: 

£-(^)"~\ whence T' = T Ql 



The final temperature may also be found from the formula : 

F 
P~ 



r = t (- 



T and T' being absolute temperatures in each case. 

The compression curve of an air-indicator card may be con- 
structed as follows, P V being the pressure and volume at one point 
of the curve andP r V the pressure and volume corresponding to any 
other point. Designating the index number of the curve by x: 

= ■==■ . rrom this. 



P' v\ 

log. f * 

p \ / v \ 8 V p' 

log. ( —A = x lose. I -rr J 5 whence, x = 



P 7 -- -&■ w J ' , {V 



"• (t) 



In considering an air card, it should be observed that the sev- 
eral lines have significations entirely different from those of a steam 
card. Referring to Fig. 72, which represents an ideal card: 
A B is the admission line, B C the compression line, C D the de- 
livery or discharge line, and D A the re-expansion line. The last- 
named line represents the effect of the re-expansion of the air filling 
the clearance space in the cylinder, on beginning a stroke (see latter 
part of Chapter X). Comparing the lines of the air and steam 
cards, they are found to be reversed, thus : 

Air Card. Steam Card. 

Admission line. Back-pressure or exhaust line. 

Compression line. Expansion line. 

Delivery line. Admission line. 

Re-expansion line. Compression line. 



PERFORMANCE OF AIR COMPRESSORS 141 

The elements of an air-indicator card, together with the work 
done, as represented by the several lines and areas, will be further 
elucidated by referring to Fig. 73. 

In this analysis the compression is supposed to be done adia- 
batically. 

Delivery Line 



B Admission Line 

Fig. 72. 

Let A D = normal atmospheric line at sea-level. 

A G = p = corresponding atmospheric pressure, acting behind 
the piston at the beginning of the stroke (neglecting 
valve resistances and effect of clearance of previous 
stroke). 
G E = A D = length of stroke of piston. 
A B = adiabatic compression curve. 
BC = delivery line. 
At the point B the useful work of compression ceases ; during 
the remainder of the stroke the volume of compressed air v', at 
the absolute pressure p f , is being forced out of the cylinder through 
the delivery valves. 

The area A B FG = the absolute work of compression. 
The area B C E F = the absolute work of deliverv. 
The sum of these areas represents the total absolute work 
(that is, on the basis of absolute pressure) done during compres- 
sion and delivery. 



142 



COMPRESSED AIR PLANT FOR MINES 



Area ADEG = work done for the entire stroke by atmos- 
pheric pressure behind the piston. 

Area A B H = net work of compression. 

Area BCD H = net work of delivery. 

Area A B C D A = total net work for entire stroke. 

From this analysis another method may be derived for calcu- 
lating the theoretical horse-power required for compressing air. 




T 



_i_ 



Fig. 73. 

It will often be found useful, when a table of temperatures of com- 
pression is available. 
Let w= weight of a unit volume (1 cu. ft.) of free air = .0765 lbs. 

C p = specific heat of air at constant pressure = .2 37 5. 

C v = specific heat of air at constant volume = .1689. 

C n 

Whence, ~=n= 1.406, and = 3-4°- 

C„ n — 1 

J = Joule's heat unit, taken as 772 ft. lbs. The work rep- 
resented by the area ABH = 

J X w X C v (T'-T) -p (v-l/). 



PERFORMANCE OF AIR COMPRESSORS 143 

Also, the work done during delivery = BC DH = if (jf — p) . Hence, 
the total net work for one stroke of the piston 

= area ABCDA = JXwXC„ (T'-T) - (pv-pif). 
If C P be substituted for C v , then pv = p'v', according to the 
general equation for air compression and the total work, W = 

JX^XC, (T'-T). 
Substituting for J, w, and C P , their constant numerical values : 
W = 14.01 (T'-T), 
from which, for an air temperature of 6o° F., at sea-level, 



[?-] 



By referring to the last column of Table VII and remembering 
that T and T' are absolute temperatures, i.e., thermometric tem- 
peratures plus 459 F.. the horse-power required for compressing 
one cubic feet of free air adiabatically to any gauge pressure may 
readily be calculated. 

Other expressions, for the mean effective pressures, may also 
be deduced from what precedes : 

w / T' \ ( T' \ 

M.E.P. for the entire stroke = p ( — — 1) =3.46^ (— — 1 ) 

M.E.P. during delivery = — (p'-p). 

The M.E.P. for compression only is found by taking the dif- 
ference between the pressures calculated by the last two formulas. 

The results obtained from the above expressions for work and 
mean effective pressure are theoretical To find the actual horse- 
power required, allowances must be made for the several losses 
experienced in the operation of the compressor, as already 
set forth. 



CHAPTER XI 

AIR RECEIVERS 

On being discharged from the compressor cylinder the air is led 
into a receiver before passing to the air main. Users of compressed 
air have been slow to realize the important part played by the air 
receiver in the economical operation of compressors, and until 
recently insufficient attention has usually been given to questions 
of its capacity, design, and position relative to the compressor. 

In its common form the receiver consists merely of a cylindrical 
shell of steel plate, resembling a steam boiler without tubes or 
flues. It is provided with pipe connections to the compressor and 
air main, a pressure gauge, safety-valve, drain cock, and man-hole. 
The receiver may be set vertically or horizontally, the vertical form 
being generally preferable, as it occupies less floor space (Fig. 74). 
Another design, which may also be employed as an intercooler, is 
illustrated in Fig. 45. The cubic capacity of the receiver 
should be properly proportioned to the size of the compressor. 
The dimensions range from, say, 24 ins., diameter by 4 or 6 ft. 
long, up to 48 or 60 ins. by 14, 16, or 18 ft., the largest sizes hav- 
ing a capacity of from 200 to nearly 400 cu. ft. Receivers are 
usually built to stand a test of 165 lbs. cold-water pressure, for 
working under pressure of 100 to 120 lbs., higher pressures than 
this being rarely necessary in ordinary practice, such as mine 
service. The shells are single-riveted on circular seams and, ex- 
cept for small sizes, double-riveted on longitudinal seams; the 
heads being dished or hemispherical. To produce the best results, 
the receiver should be placed close to the compressor, or in any 
case not more than 40 to 50 ft. distant. A large horizontal re- 
ceiver is shown in Fig. 75. 

The principal functions of an air receiver may be summarized 

144 



AIR RECEIVERS 



145 



as follows: (1) to eliminate the 
pulsating effect of the strokes of 
the compressor piston and pre- 
vent rapid fluctuations of press- 
ure; (2) to minimize the fric- 
tional loss attending the flow of 
air through the lines of piping; 

(3) to serve in some degree as an 
equalizer and reservoir of power ; 

(4) to cool the air as thoroughly 
as possible before it passes into 
the main, thus causing it to de- 
posit a part of its moisture in the 
receiver, whence it is drained off. 

Regarding the first point, the 
volume of the receiver should be 
sufficiently great in proportion 
to that of the compressor cylin- 
der to prevent any material rise 
or fall of pressure in the receiver 
by the incoming volume of air 
forced into it at each stroke. If 
the air were discharged directly 
into the main, large fluctuations 
of pressure would occur, accom- 
panied by periodic acceleration 
of flow of the air. This would 
not only increase the frictional 
resistance in the pipe, but at the 
end of each stroke the compres- 
sor piston would have to force 
the air out of the cylinder 
against a pressure momentarily 
greater than the normal. A 
loss of power would thus be 
caused, and the variation of the 




Fig. 74. — Vertical Air Receiver (Nor- 
walk Iron Works Co.). 



10 



146 



COMPRESSED AIR PLANT FOR MINES 



work done throughout the stroke of the piston would be increased. 
The violence of the discharge pulsations is obviously greater with 
a single cylinder than a stage compressor, working to the same 
air pressure, because the total discharge must take place from the 
cylinder of larger diameter in a smaller proportion of the length of 
stroke than is the case with the high-pressure cylinder of a stage 
compressor. In the latter the delivery valves open earlier in the 
stroke, and the air pipe is about one-half the diameter of the cylinder. 



FOR * 
SAFETY VALVE 




Fig. 75. — Horizontal Receiver-Aftercooler (Ingersoll-Rand Co.). 

The second function of the receiver is best fulfilled by placing 
an auxiliary receiver near the point at which the compressed air is 
used. Just as the receiver at the compressor diminishes the momen- 
tary rise of pressure in the main caused by each stroke of the pis- 
ton, so a second receiver close to the engine or machine using the 
air will prevent a drop of pressure as each cylinderful of air is 
drawn off. By reducing the fluctuations of pressure the two re- 
ceivers maintain a practically constant flow of air through the main 
connecting them and the friction and loss of pressure are thus 
minimized.* 

For mine service the second receiver would usually be placed 
somewhere underground. This arrangement is always advan- 
tageous when the air main is of great length. Underground re- 
ceivers are not often used for air drills alone, but they become a 

* Other questions relating to the flow of air in pipes, frictional losses, etc., are 
discussed in detail in Chapter XVI. 



AIR RECEIVERS 147 

necessity when large machines, such as pumps and hoists, are run 
by compressed air. They are useful, moreover, in permitting a 
further deposition of moisture from the air, thus rendering the air 
dryer and more suitable for use in expansive-working engines. To 
be most effectual in accomplishing this, the underground receiver 
should be placed at the point in the pipe line where the air has 
reached its lowest temperature — a consideration not always' con- 
sistent with the local conditions. 

Underground receivers are usually similar in construction to 
those installed near the compressor. Sometimes, however, as for 
example at the Mansfeld copper mines, Germany, another mode of 
construction has been satisfactorily adopted. A chamber is ex- 
cavated in the rock, all loose stone removed, and the walls cemented 
tight. The chamber is closed by a brick dam composed of two 
parallel walls, with a two-inch layer of cement between them. In 
the dam are set a cast-iron man -hole with suitable cover, several 
pipes for connecting with mains to the various working places, and 
a drain pipe and cock close to the floor. The latter is opened from 
time to time, to blow out the accumulated water and sediment. 
A pressure gauge is attached to the man-hole cover. Such reser- 
voirs may be built to cost much less (for large sizes) than ordinary 
shell receivers of equal capacity.* 

The third function of the receiver is apt to be misunderstood 
or exaggerated. While it is true that it acts to a limited extent as 
a reservoir of power; yet, to be of much practical service in this 
respect, its capacity must be very large. 

For example, take a 2C-in. compressor, working at 60 lbs. 
pressure to supply air for a regular consumption. To enable the 
receiver to meet the demand for only 1 minute after the com- 
pressor is stopped, and not have the pressure fall more than 15 lbs., 
it would have to be 5 ft. diameter by 50 ft. long. Again, if the 
compressor were running at a constant speed and the demand for 
air should suddenly increase 25 per cent. — as might happen in 

* Zeitschrift }iir das Berg-, Hiitten-und Salinen-Wesen, Vol. XLI, p. 119. A 
receiver of the kind mentioned was built at Mansfeld for about one-third the cost of 
an equivalent steel receiver. 



148 COMPRESSED AIR PLANT FOR MIXES 

starting several more machine drills — a receiver of the size men- 
tioned could meet the extra demand only 4 minutes.* It is thus 
evident that while a receiver is useful as an equalizer within cer- 
tain limits, yet, unless it be large, the pressure might quickly run 
up to an unreasonable amount in case of an unexpected decrease 
in consumption of air. Long pipes of large diameter assist in 
equalizing the flow of air, but their use does not preclude the 
necessity of receivers. It is much cheaper to employ piping of 
moderate size, in connection with a receiver of generous di- 
mensions. 

Frobably the most important office of the receiver is to cool 
the air before it passes into the main. In recent years much more 
attention than formerly has been given to this point. The velocity 
of flow of the air coming from the compressor is greatly reduced on 
entering the relatively large volume of the receiver; it is cooled 
somewhat at the same time, and caused to deposit a part of the 
moisture in suspension, which otherwise would be conveyed into the 
system of piping, and thence to the machines using the air. It is 
intended that the receiver shall be of sufficient capacity to drain the 
air as thoroughly as is economically practicable. But in the or- 
dinary sizes of shell receiver the results are usually quite imperfect, 
because the air passes through too rapidly to permit any large drop 
in temperature. The inlet and outlet pipes of the receiver should 
be placed in proper relative positions. If at opposite ends, and 
especially if these pipes point toward each other, a strong through 
current is caused, which reduces the usefulness of the receiver. A 
large part of the entering volume of air passes out again without 
having had time to cool or to drop much of its entrained moisture. 
One mode of arranging the pipe connections is to place the inlet on 
one side, near the end of the receiver, while the outlet is at the 
opposite end, in the middle of the head. The air is thus forced to 
change its direction of flow. Or, as in Fig. 74, both pipes may be 
connected near the top, the outlet pipe being carried through the 
receiver nearly to the bottom, where the air is likely to be slightly 
cooler (and dryer) than at the top. As the inlet pipe shown in this 

* Xonvalk Iron Works Catalogue, 1906, p. 63. 



AIR RECEIVERS 149 

case is connected tangentially to the periphery of the receiver, a 
rotary motion is imparted to the body of air, so that each particle 
remains longer in the receiver and under its cooling influence. 
Some receivers are provided with baffle-plates for the same pur- 
pose, as in Fig. 75. With wet compressors a large amount of 
moisture is carried into the receiver ; even in dry compressors some 
water collects from the natural moisture of the atmosphere, 'espe- 
cially in warm weather. Part of the lubricating oil carried over from 
the compressor cylinder is also deposited in the receiver. At inter- 
vals, according to atmospheric and other conditions, the water 
and oil are drained off by means of the cock provided for the 
purpose. 

Another result of cooling in the receiver may be noted. A 
receiver of ample size, placed close to the compressor, tends in 
some degree to economize power; because, whatever cooling is 
accomplished reduces proportionately the temporary increase of 
pressure due to the heat of compression. Hence, the piston con- 
sumes somewhat less power in forcing the air out of the cylinder 
against the receiver pressure than if the air were left to cool 
gradually in a long length of piping. As the heat of compression 
must be lost in any case before the air is used, this saving is worth 
while, however small it may be, since it is produced without cost and 
incidentally to the normal operation of the receiver. 

This has of late led to the employment of what are called "re- 
ceiver after-coolers." They are practically identical in construc- 
tion with the large tubular intercoolers shown in Figs. 36 and 39.* 
The shell contains a series of water-cooled tubes, between which 
the air is caused to circulate before passing to the larger outer por- 
tion of the receiver, whence it is discharged into the main. Having 
a sufficient volumetric capacity and cooling area of tubes, this type 
of receiver cannot fail to be more efficient as an after-cooler and the 
benefits of employing a receiver are more fully realized. 

* These are referred to, in the latter part of Chapter VI, as being applicable as 
intercoolers for stage compressors. See also an article by Frank Richards, in 
Compressed Air, Jan., 1907, p. 4329. 



CHAPTER XII 

SPEED AND PRESSURE REGULATORS FOR COM- 
PRESSORS 

If the consumption of compressed air were constant, no more 
regulation of the compressor's speed and power would be required 
than that furnished by an ordinary governor for the steam end, to 
take care of fluctuations in boiler pressure or accident to some part 
of the mechanism. But the conditions under which most air com- 
pressors operate make it necessary to provide for running econom- 
ically even when there are wide variations in the rate at which the air 
is used. In event of a sudden temporary decrease in consump- 
tion, the compressor must be slowed down, the alternative being 
to blow off air at the receiver safety valve, just as steam would be 
blown off under similar circumstances from a boiler. As a cubic 
foot of compressed air, however, costs more than a cubic foot of 
steam, the air cannot be allowed to go to waste at a safety valve. 
The compressor must be furnished with some device for coor- 
dinating the quantity of steam admitted to the steam end with the 
variable air pressure in the receiver, thereby regulating the piston 
speed in accordance with the demands upon the air end. Further- 
more, it is not enough to provide only for varying the speed of the 
compressor. At times, the consumption of air may cease entirely 
for a short period, and, to avoid the necessity of bringing the com- 
pressor to a standstill, provision should be made for unloading the 
air end. When this is done useful work stops for the time being, 
the compressor consuming only enough steam to turn its centers 
and overcome friction of the moving parts. 

Numerous regulating and unloading mechanisms have been 
devised, so that instead of requiring the almost constant attendance 
of an engineer at the throttle, the modern air compressor operates 

150 



SPEED AND PRESSURE REGULATORS FOR COMPRESSORS 151 

automatically under the widest variations of load. As these useful 
devices differ greatly in design, the subject will best be illustrated 
by giving a few examples in detail. They may be classified under 
two heads: (1) speed governors and pressure regulators; (2) un- 
loaders for the air cylinders. 

Speed Governors and Pressure Regulators. Speed governors 
are usually of the ordinary centrifugal or fly-ball type, and may be 




Fig. 76. — Clayton Governor and Pressure Regulator. 



applied to the steam end of the compressor merely to regulate its 
speed, as in case of a steam engine; or their action may be so modi- 
fied and controlled by the changing receiver pressure as to produce 
a combined speed and pressure regulation. The air cylinder is not 
completely unloaded at any time, the compressor being simply 



152 COMPRESSED AIR PLANT FOR MINES 

speeded up or slowed down in conformity with the rate at which 
the air is used. 

The pressure regulator of the fly-ball governor type may be 
illustrated by Fig. 76 (Clayton governor). The stem of the throttle 
valve, h, which is inserted in the steam pipe, connects with the 
spindle of the ball governor, by which the speed of the compressor 
is limited and controlled. At p is shown the bevel gearing for 
operating the governor, a small pulley being mounted on the gear 
shaft and driven by belt from the crank-shaft of the compressor. 
By means of the weighted lever, i, and the small air cylinder, j, the 
action of the ball governor is modified by the air pressure in the 
receiver. Air from the receiver enters the cylinder, j, through 
the pipe, k, and when the pressure exceeds its' assigned limit, raises 
the piston and weight, and shuts off steam by forcing down the 
throttle valve, //, the pressure of the lever being applied at the point, 
/. The governor may be adjusted to its work by the spring and 
thumb-screw, m, acting on the small lever, n, which tends to keep 
open the throttle against the downward pressure of the weighted 
lever, i, upon the valve stem. The spring, 0, is introduced to ease 
the drop of the weight when the air pressure falls. 

Other designs, similar in general principle but varying in 
many details, are used on the Ingersoll-Rand, Sullivan, Franklin, 
McKiernan, American, and other compressors, when steam- 
driven and of the straight-line or duplex type. The Sullivan 
speed and pressure regulator, as supplemented by an unloading 
attachment, is described hereafter. 

An entirely different form of governor is the Norwalk (Fig. 
77). A balanced throttle valve, a, is placed in the main steam 
pipe, and above it is set a small air cylinder, b, the piston rod of 
which is a prolongation of the valve stem, c. At the side of the 
cylinder, b, is a spring safety valve, d, connected by a pipe, e, with 
the receiver, or with the air main leading to it. By means of a 
hand-wheel, /, on the safety valve, the spring is adjusted so that the 
air will lift the valve, and pass through it, at any desired pressure. 
When the receiver pressure exceeds this limit the safety valve, d, 
rises and allows air to pass under the piston in the small cylinder, 



SPEED AND PRESSURE REGULATORS FOR COMPRESSORS 1 53 

b, raising it and partly closing the throttle. If no escape were 
provided the piston would be forced at once to the top of the cylin- 
der. To regulate its movement and prevent shutting off the 
steam completely, a very narrow vertical slot is cut in the side of 
the cylinder. As the piston rises, more and more of this slot is un- 
covered and furnishes an escape for the air passing into the cylim 




Fig. 77. — Norwalk Pressure Regulator. 



der. The slot being very narrow, a slight difference in the quan- 
tity of air causes the piston to assume a high or low position. In 
this way the throttle is moved, controlling the admission of steam 
and the compressor speed. As the air pressure falls the valve 
begins to open again. To prevent the small piston from rising 
too far and stopping the compressor by completely closing the 
throttle, a screw stop, g, is set in the top of the regulating cylinder, 



154 



COMPRESSED AIR PLANT FOR MINES 



b. This can be so adjusted by hand that, when the small piston 
has reached the top of its stroke, just enough steam is admitted 
by the throttle to keep the compressor in motion. 

In another form of this governor, shown in vertical section in 
Fig. 78, the fine slot in the little cylinder, B, is replaced by a tapered 




Fig. 78. — Norwalk Pressure Regulator. 

recess in the stem or piston rod of this cylinder, at the point where 
it passes through the lower head (indicated at R, in the small cut 
to left of main figure). As the piston in this cylinder is forced 
upward by the air pressure the area of the opening formed by 
the slotted stem furnishes a graduated escape for the air, and so 



SPEED AND PRESSURE REGULATORS FOR COMPRESSORS 1 55 

regulates the small piston's movement, and through it the throttle 
valve. 

The Ingersoll-Rand Co. also makes a steam regulator in which 
the stem of the main throttle is prolonged to the piston of a small 
horizontal air cylinder, attached to the side of the throttle. This 
piston is moved by air pressure conveyed through a quarter-inch 
pipe from the receiver. 

Another governor (Clayton) is shown in Fig. 79. The 
throttle valve, a, is provided with a lever and weight, b, connected 




Fig. 79. — Clayton Pressure Regulator. 

with the valve stem, c. Close alongside of the throttle, and for 
convenience clamped on the steam pipe, d, is a small air cylinder, 
e, the upper end of whose piston rod is pinned at / to the weighted 
lever. Entering the bottom of this air cylinder is a small pipe, 
g, from the air receiver. When the pressure in the receiver ex- 
ceeds the assigned limit the weighted lever is raised and, partially 
or wholly, closes the steam throttle, a. Then, when the air press- 



156 COMPRESSED AIR PLANT FOR MINES 

lire has been reduced by the slowing down of the compressor, and 
by consumption of air from the receiver, the weight falls and re- 
opens the throttle. 

With governors and regulators of the type represented by the 
preceding examples, the operation and control of the compressor 
is not automatic under all conditions, but they answer the pur- 
pose for some kinds of service. In case no air is drawn from the 
receiver, the compressor slows down until it just passes its 
centers; then, in most of the designs, if the pressure continues to 
rise, a little air will blow off at the receiver safety-valve, or the 
compressor may be stopped completely by closing the steam 
throttle. 

Air-Cylinder Unloaders. These are designed to exercise com- 
plete automatic control over the compressor, when the latter is 
belt-driven; and also for steam-driven compressors when used in 
conjunction with a governor. The throttle is first nearly closed 
(in steam compressors), as the consumption of air decreases; 
then, if it ceases altogether, the unloading mechanism either shuts 
off the intake air or else opens the discharge valves, thus ad- 
mitting air at receiver pressure to both ends of the cylinder. In 
either case the pressures on opposite sides of the piston are bal- 
anced and all useful work ceases, though the compressor contin- 
ues to turn its centers, taking only enough steam to overcome 
friction. 

The Rand " Imperial " unloader, for small compressors 
driven by belt or direct-connected electric motor, furnishes an 
example of this type of regulator (Fig. 80). It is inserted in the 
intake pipe, and shuts off the air from the inlet valves when the 
receiver pressure rises above the set limit. In the cut the passage 
of the intake air is shown by the arrows. The small chamber 
(6c) is connected by a J- in. pipe with the receiver. As the 
pressure increases, the piston (57) moves against the resistance 
of the spring (56), admitting receiver air, through the small ports 
on the left of the piston, to the lower side of the plunger valve 
(61). On reaching its seat this plunger closes the intake to the 
compressor cylinder. The resistance of the spring (56) may be 



SPEED AND PRESSURE REGULATORS FOR COMPRESSORS 157 

adjusted by the screw-plug (55), for any required working press- 
ure. As the receiver pressure falls again, on increased con- 
sumption of air, the spring forces down the piston (57). This 
closes the lower small air port, leading to the under side of the 




SECTIONAL VIEW 
RAND IMPERIAL UNLOADER 



K Pipe 
\ from Receiver 



Fig. 80. 



plunger valve (61), and at the same time opens the upper hori- 
zontal port, connecting with the open screw-plug (55). The 
air below the plunger valve is thus exhausted, causing the latter 
to fall, thus reopening the intake passage. The useful work of the 
compressor is then resumed. 



158 



COMPRESSED AIR PLANT FOR MINES 



An unloader similar to the above is used for some of the Ailis- 
Chalmers compressors. The Ingersoll-Rand Co. makes an 
automatic " choking " controller, -which is applied to the intake pipe 
of the piston-inlet compressor. It is adjustable for any desired 



Oil Got. Here 




Fig. 8i. — Sullivan Governor and Unloader. 



limit of pressure by a weighted lever, and may be used for all 
forms of steam-driven compressors. 

A combined governor and pressure regulator, with unloading 
attachment, as employed by the Sullivan Machinery Co., will 
illustrate a type of compressor regulator that has been adopted by 
several builders, though with many variations in details of design 
(Fig. 81). It may be used with straight-line or duplex, steam- 
driven compressors. The split-ball governor (n), belt-driven 
from the crank-shaft to the pulley (20), accompanied by the 



SPEED AND PRESSURE REGULATORS FOR COMPRESSORS 1 59 

tightener (43), controls the steam throttle (3). Connected 
with the governor spindle and throttle valve stem, at 28, is a 
lever (25), the position of which is influenced by the centripe- 
tal action of the set of springs (31,32, and 26). By screwing 
up or down the hand-wheel and speeder screw (5), this system of 
springs (and with them the governor) is set to run the compressor 
at any desired speed. The other element of the governor is the 
air-pressure device, which, by the position of the plunger in the 
small air cylinder (18), causes the springs to be brought into action 
in the order of their strength, thus producing movement of the 
lever (25). 

The pressure device is connected with the air receiver by the 
union valve (33), admitting air to the little cylinder (27), the piston 
of which operates a needle valve. This valve is held closed 
against any desired minimum air pressure by the adjustable 
weight (36) and the regulating screw and spring (37 and 38). 
When the receiver pressure rises above the normal, it opens the 
needle valve and admits receiver air to the cylinder (18). As the 
pressure increases, the plunger in (18) rises against the counter- 
spring (26) and through the lever (25) tends to close the main 
steam throttle (3), thus slowing the compressor. Total stoppage 
is prevented by screwing down the nut of the stop-screw (23), so 
as to limit the upward movement of the pressure plunger. This 
plunger is designed to act intensively, being so proportioned that 
a variation of only 2 or 3 lbs. receiver pressure is multi- 
plied to 40 lbs. in its action on the governor. In this way a sen- 
sitive control is produced within narrow limits of working air 
pressure. To prevent violent movements of the pressure ele- 
ment, in case of sudden changes of receiver pressure, the 
plunger in (18) is provided with an oil dash-pot. 

A somewhat similar pressure regulator and unloader is used 
on the Franklin compressor.* 

Another unloader, applicable to straight-line and duplex 
compressors, and in a modified form to stage compressors also, is 
the Ingersoll-Sergeant. It differs materially from the unloaders 

* Mines and Minerals, May 1905, p. 504. 



i6o 



COMPRESSED AIR PLANT FOR MINES 



previously described, in controlling the action of the discharge, 
instead of the inlet valves. The principles of its construction 
and operation will be understood by reference to the longitu- 
dinal section in Fig. 82. The most recent design of this unloader 
differs in some details from that shown in the cut, but its mode 
of working is unchanged, and many are in use. 

A weighted plunger, a, working in a small cylinder, is at- 
tached for convenience to the shell of the compressor cylinder. 




To Discharge Valve 

Fig. 82. — Ingersoll-Sergeant Regulator ami Unloader 



Connection with LJ 
Discharge Valve 



From the chest in which a is set there are four pipe connec- 
tions as shown : b leads to a balanced throttle valve in the 
main steam pipe, c connects with the air receiver, and d and 
e with one or more discharge valves at each end of the cylin- 
der. The stem of the steam throttle, /, is a continuation of 
the piston rod of a small horizontal air cylinder, g, which is 
attached to the side of /. Behind the piston of this little cylin- 
der enters the air pipe, b. When the pressure in the receiver 
becomes too great the safety valve, a, rises, and exhausts the air 
behind the two discharge valves which are connected with the 



SPEED AND PRESSURE REGULATORS FOR COMPRESSORS l6l 

pipes, d and e. This admits air at receiver pressure into each end 
of the compressor cylinder, thus balancing the pressure on the 
two sides of the piston and unloading the engine. At the same 
time the air in the little cylinder, g, is also exhausted, so that the 
throttle valve, /, moves to the right and admits only enough steam 




Fig. 83. — Laidlaw-Dunn-Gordon Air Governor. 



11 



1 62 COMPRESSED AIR PLANT FOR MINES 

to keep the compressor slowly turning. When the compressor is 
thus unloaded no work is done; the air is merely circulated through 
the pipes, d and e, from one end of the cylinder to the other, until 
more air is drawn from the receiver and the pressure reduced. 
Then the safety valve, a, closes and the pipes, d and e, are again 
filled with compressed air. The steam throttle is also forced open 
by the pressure through the pipe, b, and compression goes on 
regularly. The admission and discharge lines of an air card from 
a compressor thus unloaded form practically a single horizontal 
line, at a height above the atmospheric line representing the 
receiver pressure. 

For steam-driven compressors of the Corliss type, as built by 
the Ingersoll-Rand, Nordberg, Laidlaw-Dunn-Gordon, Sulli- 
van, Allis- Chalmers, and some other companies, the air-press- 
ure regulators act in conjunction with ball or other centrifugal 
governors. All of them control the operation of the compressor 
by acting upon the expansion gear of the steam end and chang- 
ing the point of cut-off. 

The Laidlaw-Dunn-Gordon governor (Fig. 83) may be taken 
as an example. Air is admitted from the receiver to the small 
cylinder, A, the piston of which is weighted, as shown. The 
action of the lever, B, supporting the weight is adjusted by the 
coil spring, C. This lever is linked to a floating lever, D, pinned 
to the vertical side rods of the ball governor. D is connected by 
the link E to the bell-crank, F, the lower arm of which is pin- 
connected to the long horizontal rod, G. By this system of levers, 
the movement of G, and through it the point of cut-off of the Cor- 
liss steam gear, is under the combined control of both ball gov- 
ernor and of the receiver pressure as influencing the position of 
the piston of the cylinder A. The arm, H, is pivoted at the foot of 
the governor post. Connected to it are the cam, I, and the idler 
pulley, J, which rests on the governor belt. In case the belt 
breaks, the idler pulley falls and the cam allows the governor 
to drop, thus shutting off steam and preventing the compressor 
from racing. The designs of governors of this type are worked 
out in a number of different ways. 



SPEED AND PRESSURE REGULATORS FOR COMPRESSORS 1 63 

Another example of governor is that employed on the constant- 
speed, variable-delivery compressor, built by the Nordberg 
Manufacturing Co. It is for motor-driven machines, with 
Corliss air valves, and operates by closing the inlet valve before the 
stroke is completed.* During the remainder of the forward 
stroke, the air already admitted to the cylinder is expanded below 
atmospheric pressure, and is then compressed on the -return 
stroke. This is practically equivalent to varying the working 
length of the stroke. 

* American Machinist, Aug. 22d, 1907, 



CHAPTER XIII 

AIR COMPRESSION AT ALTITUDES ABOVE SEA- 
LEVEL 

Because of the diminished density of the atmosphere, air 
compressors do not produce the same results at high altitudes 
as at sea-level. Their effective capacity is reduced because a 
smaller weight of air is taken into the cylinder at each stroke. 
It is necessary, therefore, to modify the figures relating to the 
capacity and performance of compressors, as set forth in the 
latter part of Chapter VII. This matter is of special importance 
in connection with mining operations, because of the large num- 
ber of mines situated in elevated mountain regions. The rated 
capacities of compressors, in cubic feet of air, as given in the 
makers' catalogues, are for work at normal atmospheric pressure, 
and due allowance must be made for decreased output at eleva- 
tions above sea-level. This reduction in output, which is usually 
also tabulated in handbooks and catalogues, should receive 
due consideration in order to avoid serious errors. For example, 
the volume of compressed air delivered at 60 lbs. pressure, at 
10,000 ft. elevation, is only 72.7 per cent, of the volume de- 
livered at the same pressure by the same compressor, at sea -level. 
In other words, a compressor which at sea -level will supply power 
for 10 rock-drills, will at an elevation of io,oco ft. furnish air 
for only 7 drills. 

The foregoing statement relates only to the volumetric capac- 
ity of the compressor. It must be remembered that the heat of 
compression increases with the ratio of the final absolute pressure 
to the initial absolute pressure. As this ratio increases with the 
altitude, more heat will be generated by compression to a given 
pressure at high altitudes than at sea-level. This additional heat 

164 



AIR COMPRESSION AT ALTITUDES ABOVE SEA-LEVEL 1 65 

temporarily increases the pressure of the air in the cylinder, while 
under compression, and more power is therefore required to com- 
press and deliver a given quantity of air. The corresponding loss 
of work, due to the subsequent cooling of the air in receiver and 
piping, also increases with the altitude. 

Contrary to a common impression, the volume of air de- 
livered by a given compressor does not bear a constant ratio ,to the 




Fig. 84. 



barometric pressure, but at different altitudes this volume de- 
creases slower than the barometric pressure. This relation may 
be shown as follows.* Two ideal indicator cards are represented 
in Fig. 84, one of a compressor working at sea-level, with an in- 
itial pressure P 1; the other at an altitude with a lower initial 
pressure P 2 . The initial volume V and the final gauge pressure P, 
are the same for both compressors, P 3 and P 4 being the respective 
final absolute pressures. V x and V 2 are the final volumes, corre- 
sponding to the dotted isothermal curves, these volumes being 
taken as the basis, because they are those to which the com- 

* The general method of demonstration here given, together with Fig. 84 and 
accompanying table, are taken by permission from an article by F. A. Halsey, in 
American Machinist, June 2d, 1898, p. 27. 



1 66 COMPRESSED AIR PLANT FOR MIXES 

pressed air will eventually shrink on losing the heat of com- 
pression. From the theory of air compression, 

VP, - V^, or ^- = p* (i) 

and VP, = V 3 P 4 , or^ = £ (2) 

v 2 r 2 

But since P 3 = P a + P, and P 4 = P 2 + P, equations (i) and 
(2) may be written: 

V P, + P P 

v --T~- = I "F (3) 

* 1 *- 1 * 1 

v p - p p 

and ir = ^—^- = 1 - £- (4) 



V, P, P 



Dividing equation (3) by equation (4) : 
P^ 

y" = p-»or^ :\ 2 ::i-f — :i+ — (5) 

1 + p- 

This gives an expression for the ratio between pressure and 
volume at sea -level and for any altitude above sea-level, of which 
the corresponding barometric pressure is P 2 . Thus, let P 2 = 
10 lbs., P = 90 lbs., and \\ (from Table VII, page 139) = 0.1404 cu. 
ft. By substituting these quantities in equation (5), V 2 is found 
to be 0.0999, or approximately 0.1 cu. ft. 

In Table VIII, columns 4 and 5, are given the relative 
volumetric outputs, at gauge pressures of 70 and 9c lbs. 
of a compressor working at different altitudes, the figures being 
percentages of the normal output at sea-level. These per- 
centages have been derived by Mr. Halsey from equation (5), a 
constant loss of initial pressure of 0.75 lb. being assumed to 
allow for the resistance presented by the inlet valves, to which 
reference has been made in another chapter. That is, for 
practical purposes the sea -level atmospheric pressure is taken as 
14, instead of 14.7 lbs. The other columns show the mean 
effective pressures and indicated horse-powers, corresponding to 
different altitudes, up to 15,000 ft., which will be found con- 



AIR COMPRESSION AT ALTITUDES ABOVE SEA-LEVEL 167 

venient for reference. It should be noted from the figures in 
columns 4 and 5, which are for the ordinary range of press- 
ure employed in mining, that, though there is a difference of 
20 lbs. between the two gauge pressures, yet the outputs at dif- 
ferent altitudes vary only by a few thousandths and may 
often be neglected.* Wide differences, however, occur in the 
columns of mean effective pressures and horse-powers. 

Table VIII 



-t-> 
0> 


Barometric 








Cubic Feet Pis- 


Cubic Feet 


0) 

U-i 




Relative Out- 
put for Gauge 
Pressure. 


M. E. P. for 
Gauge Pressure. 


ton Displace- 
ment perl. H.P. 
for Gauge Pres- 


Compres 

Air per I. 

for Ga 


>sed 


<g 


<r> ?? 


<v 


H. P. 

age 


=3 
•*■> 


<v ii 


Pounc 

per Squ 

Inch. 








sure. 


Pressure. 


< 


70 lbs. 


90 lbs. 


70 lbs. 


90 lbs. 


70 lbs. 


90 lbs. 


70 lbs. 9 


lbs. 


1 


3 


3 


4 


5 


6 


7 
38.2 


8 
6-93 


9 

5-99 


10 

1. 144 


11 


O 


30.OO 


14-75 


I. OOO 


I. OOO 


33- 1 


801 


1,000 


28.88 


14.20 


.967 


.966 


32.6 


37-6 


7-°3 


6.09 


1. 123 


787 


2,000 


27.80 


13.67 


-935 


-933 


32.1 


3 6 -9 


7-i5 


6.20 


1. 103 


773 


3,000 


26.76 


13.16 


.904 


.900 


3i-5 


3 6 -3 


7-27 


6.31 


1.084 


759 


4,000 


25.76 


12.67 


-873 


.869 


31.0 


35-6 


7-39 


6-43 


1.065 


746 


5,000 


24.79 


I2.20 


.843 


-839 


3°-5 


35-° 


7-5i 


6-55 


1.046 


733 


6,000 


23.86 


n-73 


-813 


.809 


30.0 


34-3 


7-65 


6.67 


1.028 


720 


7,000 


22.97 


11.30 


-7«5 


.780 


29.4 


33-7 


7.80 


6-79 


I. Oil 


708 


8,000 


22.11 


10.87 


-758 


-751 


28.9 


33-i 


7-94 


6.92 


-994 


695 


9,000 


21.29 


10.46 


-731 


-723 


28.3 


32-5 


8.09 


7.06 


-976 


683 


10,000 


2O.49 


10.07 


-705 


.696 


27.8 


31.8 


8.24 


7.20 


-959 


670 


II,000 


19.72 


9.70 


.680 


.671 


27-4 


31.2 


8-39 


7-34 


-942 


658 


12,000 


18.98 


9-34 


.656 


.647 


20.9 


30.6 


8.^4 


7-49 


-925 


.646 


13,000 


18.27 


8.98 


.632 


.623 


26.3 


30.0 


8.71 


7-64 


.908 


■635 


14,000 


17-59 


8.65 


.608 


.600 


25.8 


29-4 


8.88 


7.80 


.891 


624 


15,000 


16.93 


8.32 


-5^5 


-576 


1 25.3 


28.8 


9.06 


7-96 


-875 


.613 



Owing to the increase of piston displacement per indicated 
horse-power, as shown in columns eight and nine of the table, 
some builders make the air cylinders of compressors for mountain 
work of larger diameter for the same size of steam cylinder than 
those for sea-level service. As against the losses of the air end of 
the compressor at high altitudes, there is some gain in mean effec- 
tive pressure of the steam cylinders, because the exhaust takes 

* Attention may be called to the fact that for this reason, in compressor-builders' 
catalogues, no account is taken of the gauge pressures in tables of compressor 
capacities at altitudes. 



1 68 COMPRESSED AIR PLANT FOR MINES 

place against lower atmospheric pressure. The same is true in 
part of the air exhaust of machines using the compressed air. 
But the resultant of these gains is small and cannot be given much 
weight in offsetting the losses. A large deduction, for example, 
would have to be made for the lower calorific power of a given 
fuel at high altitudes. 

The relation between compressor output and barometric 
pressure may be expressed simply in another way. Take the case 
of two compressors of the same size, one operating under an 
atmospheric pressure of, say, 14 lbs. and the other at 10 lbs. (cor- 
responding approximately to an altitude of 10,000 ft.). If the 
first compressor is producing 6 compressions, the final absolute 
pressure will be 14 X 6 = 84 lbs. or about 70 lbs. gauge pressure. 
To produce the same gauge pressure, the other compressor must 
work to an absolute pressure of 70 + 10 = 80 lbs., the number of 
compressions corresponding to which is -f-jj- — 8. From each cubic 
foot of free air the first compressor will produce -J- of a cu. ft. 
of compressed air, and the second compressor, -|- cu. ft. Hence, 
the ratio of the respective outputs of the two compressors will 
be-g- -1- -g- = f- or 0.750. As compared with this, the ratio of the 
respective barometric pressures is -J-j- =0.714. 

Mechanically Controlled Inlet Valves for High Altitudes. It 
is often stated that compressors whose inlet valves are under some 
mechanical control are of special advantage for work at altitudes 
above sea-level. While there is a measure of truth in this, the 
possible saving is necessarily small, except at considerable eleva- 
tions. The question presents itself as follows. If the valve re- 
sistance be diminished by introducing mechanical control, so that, 
under normal conditions at sea-level, the inlet air will begin to 
enter the cylinder a little earlier in the stroke, the volumetric 
capacity of the compressor is thereby increased. The loss of 
capacity due to resistance of the valve springs, etc., which has 
been assumed to be 0.75 lb., for ordinary poppet valves, is a 
constant, and therefore becomes proportionately of greater and 
greater consequence as the altitude increases, because its ratio to 
the diminishing atmospheric pressure goes on increasing. The 



AIR COMPRESSION AT ALTITUDES ABOVE SEA-LEVEL 1 69 

percentage of saving obtained by eliminating the spring resistance, 
though small at or near sea-level, therefore becomes a matter 
of importance at great elevations ; and the inlet valve which pre- 
sents the smallest resistance to the entrance of the air into the 
cylinder will be the most economical for service in high moun- 
tain regions. 

Stage-Compression at High Altitudes. According to the state- 
ment already made, the greater the altitude above sea-level 
the smaller will be the ratio between the final pressure at delivery 
and the atmospheric pressure; that is, the ratio of compression. 
In Chapter V the effect of clearance in the air cylinder was dis- 
cussed, and it is evident that the percentage loss from this cause 
increases with the altitude because the piston must advance 
farther before the clearance air has been re-expanded to a press- 
ure below the diminished atmospheric pressure. Even if it be 
questioned whether it is worth while at sea-level to adopt stage 
compression for the ordinary pressures used in mining and 
tunnelling, the case is materially altered at high altitudes. 
For example, if it be desired to produce a gauge pressure of 
75 lbs. at 5,000 ft. elevation, corresponding to an atmos- 
pheric pressure of about 12.2 lbs., 7.15 compressions 
are necessary. At sea-level this number of compressions 
would give a gauge pressure of (14.7 X 7.15) — 14.7 = 90.4 
lbs. So far as losses due to piston clearance are concerned, 
therefore, it is as reasonable to employ stage-compression 
for 75 lbs., at 5,000 ft. elevation, as for 90 lbs. at sea-level. 
In a compound compressor, too, it must be remembered that there 
is practically but one clearance space: that in the intake cylinder. 
The value of the intercooler also increases with the altitude 
because, in beginning compression at an initial pressure below the 
normal, the greater total range of pressure through which the air 
must be carried involves the production of more heat. This 
additional heat must be effectually dealt with by the cooling ar- 
rangements, if loss from this cause is to be avoided. 

Considered from both the economic and thermodynamic stand- 
points, there can be no question as to the value of stage compres- 



170 COMPRESSED AIR PLANT FOR MINES 

sion for high altitudes. There is not only a decrease in output 
and an increase in the cost of production of the air, due to the 
added power required; but, as a result of these conditions, the 
compressor itself must be larger for a given output, and therefore 
its first cost will be greater than that of a compressor of the same 
capacity, working under normal atmospheric pressure. Hence, 
by introducing stage compression a larger percentage of saving 
is possible at high altitudes than at sea-level. 



CHAPTER XIV 

EXPLOSIONS IN COMPRESSORS AND RECEIVERS 

Explosions in air compressors and receivers occur with suf- 
ficient frequency to demand careful attention. Though they are 
unquestionably attributable to ignition of volatile constituents 
of the lubricating oil, the immediate causes leading to this com- 
bustion are not always, nor altogether, clear. It is found, however, 
that explosions occur only in dry compressors, and some light 
may be thrown upon the subject by considering the conditions 
affecting the use of lubricant in these machines. In Chapter V 
attention was called to the fact that, if the cylinder temperature of 
a dry compressor be allowed to rise too high, not only does proper 
lubrication become difficult, but the oil itself may be decomposed 
by the heat. It is probable that ignition unattended by actual 
explosion is of frequent occurrence. Instances are on record 
where the discharge pipe near the compressor has become red- 
hot, and the ignition even extended into the receiver without pro- 
ducing a destructive explosion. Examination of the discharge- 
valve chests and passages, and the pipe leading from compressor 
to receiver, often reveals the presence of a black, sooty residue 
originating from decomposition of the lubricant. The volatile 
constituents of the oil thus liberated, on passing with the com- 
pressed air into the receiver, would make a mixture of air and gas 
capable of producing an explosion. The extreme violence often 
noted in such explosions is probably due in part to the high air 
pressure existing in the valve passages, discharge pipe, and re- 
ceiver. In high pressure air, combustion is always more active 
than in air at atmospheric pressure. 

A number of the recorded air-compressor explosions have oc- 

171 



172 COMPRESSED AIR PLANT FOR MINES 

curred at collieries, and the possible effects of the presence of 
coal dust in the intake air of the compressor have been carefully 
considered. A deposit of such dust in the valve passages, together 
with the sooty residue from decomposition of the oil, might as a 
result of oxidation produce a condition very favorable to an ex- 
plosion. It has been suggested that, under these circumstances, 
a spark caused by the friction of the compressor piston, if work- 
ing dry, might bring about an explosion; or, by the continual 
passage of air at a high temperature over the carbonaceous de- 
posit, spontaneous combustion might result, and ignite the in- 
flammable mixture of oil-vapor and air.* However, there are a 
sufficient number of cases where explosions have taken place at 
mines and works other than collieries to prove that such explo- 
sions are not necessarily dependent upon the presence of coal-dust 
in the intake air of the compressor. When the compressor is 
improperly situated in a room close to the boilers and coal-bins 
some coal dust might be present in the air; but though possibly 
assisting in the explosion, the quantity could hardly be large 
enough to produce by itself the observed results. 

The true cause of these explosions is undoubtedly to be found 
in the working conditions prevailing in the compressor cylinder. 
In a single-stage dry compressor an excessively high tempera- 
ture is often reached, because of improper design of the air 
cylinder, or by running too fast (as when the compressor is too 
small for its work), or by attempting to produce too high a 
pressure. The temperature of the discharge air from a single- 
stage compressor is found by the formula already given in 
Chapter X : 

in which : T and P are, respectively, the absolute initial temper- 
ature and pressure of the intake air; T' and P', the absolute final 
temperature and pressure; and n, the constant, 1.41. Under 
normal conditions near sea-level, say, when the temperature of the 
atmosphere is 70 F., P = 14 lbs., and the gauge pressure at dis- 

* T. G. Lees, Trans. Federated Inst. Mining Engineers, Vol. XIV, p. 568. 



EXPLOSIONS IN COMPRESSORS AND RECEIVERS 1 73 

charge, 80 lbs., the final temperature is found by making the 
respective substitutions, 

(o _ _i_ T . \ o 29 
— — — ) =917° F. absolute, 

or 458 F. by the thermometer. 

As calculated by this formula, the compression is supposed 
to be purely adiabatic, no account being taken of loss of heat by 
radiation or of any effect that may be produced by the water- 
jackets. As a matter of fact, but little heat can be abstracted by 
the jackets of a single-stage compressor. Air is a poor conductor, 
and the volume in the cylinder is not long enough under the in- 
fluence of the jackets to be much affected by them. In a com- 
pressor of this type the chief office of the jackets is to keep down 
the temperature of the cylinder walls and prevent the lubricating 
oil from being carbonized. It is probable, therefore, that in a 
single-stage dry compressor, even if well designed and in good 
order, the actual temperature of the air at discharge will generally 
range from, say, 375 ° to 42 5 ° F., and may often go higher— 
a statement sufficiently supported by recorded observations. 

In consideration of what precedes it is evident that the quality 
of the lubricating oil used in the air cylinder, and especially its 
flashing- and ignition-points, are matters of importance.* The 
flashing-point of ordinary cylinder oil may be taken as from 330 
t0 42 5°F. "An average of determinations on 40 samples of 
heavy oils having an average flash-point of 360 F., gave average 
burning-point of 398 F. High flash test cylinder oils, from 500 
to 560 F., gave burning-points of 6oo° to 630 F." f Common 
lubricating oils flash at about 250 F., and kerosene, sometimes 
carelessly used by compressor engineers for cleaning discharge 
valves, at 150 F. or below. In the case of one explosion the 
flash-point of the cylinder oil used was found to be only 295 F. % 

* The flashing-point of oil is the lowest temperature at which it gives off com- 
bustible vapors in sufficient quantity to be ignited by contact with flame. The 
ignition-point is the temperature to which the vapors must be raised in order to 
continue to burn. 

f Alex. M. Gow, Engineering News, March 2d, 1905, p. 221. 

J John Morison, Trans. North of England Inst. Min. Engs., Vol. XXXVIII, p. 6. 



174 COMPRESSED AIR PLANT FOR MINES 

It would appear, from a comparison of these temperatures, that 
an explosion in a compressor cylinder, directly traceable to de- 
composition of the lubricant, would be possible under normal 
conditions only when inferior, light mineral oils are employed. 

But compressors are not always in good order, nor the work- 
ing conditions always normal in other respects. Aside from the 
dangers arising from the use of low-grade lubricant, it is more 
than probable that one of the commonest causes of explosion is 
air-cylinder leakage, either of the delivery valves or past the pis- 
ton. The effects of leakage may be illustrated by citing a case or 
two. 

In 1897 an explosion took place in one of the receivers of the 
compressor at the Clifton Colliery, England.* It attracted much 
attention, and is so instructive that many of the details are given 
here. The air from the compressor passed to a series of 3 
receivers of large size, the first being 7 ft. diameter by 40 ft. 
long. While running apparently under normal conditions the 
safety-valves of the receivers suddenly began blowing off with a 
deafening roar. Flames several feet high issued at great press- 
ure from the safety-valves, and sparks were blown out at the 
joints of the 8-in. pipe leading from the compressor to the 
first receiver. The air main near this receiver was nearly red- 
hot. That the receivers did not burst was thought to be due 
to the relief afforded by the 4 safety-valves — 2 on the first 
receiver and 1 on each of the others — and to the fact that the 
underground engines driven by compressed air continued running 
for some minutes after the compressor was stopped. On ex- 
amining the first receiver, after it had cooled, it was found that, 
just below the point at which the air entered from the compressor, 
a mass of black carbonaceous matter had been deposited, from 
1 J to 2 ins. thick and 6 sq. ft. in area. On analysis this 
showed: volatile matter, 55.8 per cent., fixed carbon, 37.3 per 
cent., and ash, 6.9 per cent. The material was charred and 
had the appearance of hard vulcanite. A thin coating was 
noticed on the sides of the receiver (though only near the inlet 

*T.G. Lees, Trans. Federated Inst. Mining Engineers, Vol. XIV, pp. 555—559- 



EXPLOSIONS IN COMPRESSORS AND RECEIVERS 1 75 

pipe) and also in the air pipe itself. The other two receivers were 
free from deposit. A coating of carbonaceous matter, to a thick- 
ness, of one-quarter inch was found on the discharge valves and 
passages. The cylinder and piston surfaces were not dry and, 
though they showed signs of excessive heat, were uninjured. 

The gauge pressure was usually 60 lbs., which, with adia- 
batic compression, would correspond theoretically to a final 
temperature of 405 ° F., the temperature of the intake air from 
the engine-house being 8o°. The lubricating oil used was 
guaranteed to have a flash-point of 554 , and ignition-point of 
606 ° F. As the cylinders were water-jacket ed, the actual final 
temperature should not, in regular working, reach the above- 
named temperatures; in fact, readings previously taken from a 
thermometer in the outlet pipe showed that it usually registered 
about 350 F. It is significant, however, that on one occasion the 
mercury rose above 500 , and the thermometer tube burst. The 
temperature at the time of the explosion therefore was not known. 
Afterward a pyrometer was fixed on the outlet pipe as near as 
possible to the discharge valves, and the temperature was found 
to range generally from 400 to 420 F., varying with the speed of 
the engine and the air pressure produced. Even with these tem- 
peratures, high as they are, it would seem impossible that ignition 
of the lubricating oil could take place. It is evident that an un- 
usual increase of temperature in the air cylinders must be ac- 
counted for. 

In commenting on this accident, Mr. W. L. Saunders makes 
the following interesting remarks on explosions in compressors 
and receivers : 

" There must bean increase of temperature, or ignition would 
not take place. This increase of temperature may result either 
from an increase of pressure, which is not recorded on the gauge, or 
there may be an increase of temperature without a corresponding 
increase of pressure. Take the first instance, and it is not dif- 
ficult to understand that a compressor might deposit carbon from 
the oil in the discharge passages or discharge pipes, which in the 
course of time will accumulate and constrict the passages so that 



176 COMPRESSED AIR PLANT FOR MINES 

they do not freely pass the volume of air delivered by the com- 
pressor. Hence, a momentary increase of pressure might exist in 
the cylinder heads, or in the discharge pipe which leads from the 
cylinder to the receiver, which would surely carry with it an in- 
crease of temperature possibly exceeding the ignition-point of 
the oil. A badly designed compressor with inefficient discharge 
passages might produce this trouble. Too small a discharge 
pipe or too many angles in discharge pipes might also tend to 
produce explosions. But ignition is known to have occurred in 
a well-designed system, and other causes must be sought. We 
think many cases may be traced to an increase of temperature 
without increase of pressure; this increase of temperature can be 
excessive only when the temperature of the incoming air is ex- 
cessive. A hot engine-room from which air is drawn into the 
cylinder is a bad condition. Ignition is known to have taken 
place, however, when the temperature of the incoming air was 
normal, when the discharge passages and pipes were free and of 
ample area, so that some other cause must still be looked for. 
The only possible explanation is that the temperature of the in- 
take air is made excessive by the sticking of one or more of the 
discharge valves, thus letting some of the hot compressed air back 
into the cylinder to influence the temperature before compression. 
... It is not difficult to understand a leaky discharge valve 
letting enough hot compressed air back into the cylinder to in- 
crease the initial temperature to 200 or 300 . If so, and the 
air is being compressed to 73.5 lbs. gauge pressure we have, 
say, 300 temperature in the free air before compression, and as 
the increase is 354. 5 , the resulting temperature might be 654.5 . 
As a remedy we would suggest more care in selecting the best com- 
pressor, and in frequent cleaning of the discharge valves and 
passages. The best compressors are built so that the discharge 
valves may be readily removed. These valves should be cleaned 
once a week by the engineer, who should see that they fit properly. 
It is impossible to get good lubricating oil that is free from carbon, 
hence there will always be more or less carbon deposited on the 
discharge valves, but this must not be allowed to accumulate. 



EXPLOSIONS IN COMPRESSORS AND RECEIVERS 1 77 

Intercoolers between air cylinders and aftercoolers between 
final cylinder and receiver are also recommended. One of these 
coolers located in the discharge pipe will absolutely prevent the 
passage of flame, and will insure the protection of the mine 
against fire even though there be ignition at or near the air 
cylinder." * 

During the construction of the New York Aqueduct a fire oc- 
curred in a compressor receiver at one of the shafts. The air 
pressure was eighty to ninety pounds, and the horizontal receiver, 
set outside of the engine-house, was exposed to the hot sun. Part 
of the discharge pipe leading to the receiver had become red-hot. 
On stopping the compressor and cooling down the receiver, the en- 
tire inner surface of the latter was found to be coated with carbo- 
naceous matter at least one-eighth inch thick. Further investiga- 
tion brought out the fact that the poppet discharge valves had 
sometimes occasioned trouble by sticking, and the engineer had 
been in the habit of using a squirt-can of kerosene to cut the 
gummy material clogging them. As the kerosene had a low flash- 
point, it was quickly vaporized, and when the cylinder tem- 
perature reached a sufficiently high point the explosion took place. 

In this case, as in that previously cited, the trouble seems to 
have been caused by leakage of the delivery valves (possibly past 
the piston also), thereby raising the cylinder temperature to an 
abnormal degree. It may be added that the use of kerosene 
for cleaning gummy discharge valves is a dangerous practice, even 
when the compressor is slowed down while using it. 

The effect of leakage in the air cylinder may readily be under- 
stood from the following discussion of what takes place in the 
course of a single stroke, with the accompanying temperature 
changes. f At the beginning of the stroke, the air in the cylinder 
consists of: that which remained in the clearance spaces at the 
end of the previous stroke, that which has leaked in, and that which 
has been drawn in from the atmosphere. The clearance air, on 

* Compressed Air, July,. 1897, pp. 258-259. 

f Abstracted from a paper by E. Hill, Trans. Amer. Inst, of Min. Engs, Vol. 
XXXIV, p. 950. 
12 



178 COMPRESSED AIR PLANT FOR MINES 

re-expanding, falls from an absolute temperature of T' to T (see 
formula near the beginning of this chapter), and its effect may 
therefore be neglected. For well-designed compressors, the tem- 
perature of the air newly drawn into the cylinder may be taken as 
that of the outside atmosphere, t, though it is generally heated 
in some degree by contact with the hot inner surfaces of the 
cylinder. Finally, if L represent the volume of air leakage, then, 
since T is the absolute temperature of the entire mass of air 
occupying the cylinder at the beginning of the stroke : 

T = (i-L)/+TL (i) 

If, in the expression previously given for the temperature of the 
discharge air, viz : 

t'=t(|) 

the pressures be written in atmospheres; then, for compressors 
working at sea-level, P = i and : 

T'= T P' '■*>, whence T = ^ (2) 

Placing the values of T, in equations (i) and (2) equal to each 
other and transposing : 

,_* (p 0.29 ._ L p' 0.29) 

I - L P /02 9 

Applying this formula to a single-stage compressor, working 
to say, 7 atmospheres, or about 88 lbs. gauge, the atmos- 
pheric air being at 62 ° F., the discharge temperatures for differ- 
ent percentages of leakage will be as shown in the table. The 
temperatures for an altitude of 4,000 ft. are also given for pur- 
poses of comparison. The leakages are expressed as percentages 
of cylinder capacity. 

These possible temperatures are fully sufficient to produce an 
evolution of gas, or even decomposition of the cylinder oil, causing 
it to burn-, which would be followed by an increased liberation of 
volatile matter and the probability of explosion. 

It must be borne in mind, as pointed out by Mr. E. Hill, 
that leakage from an imperfectly fitting discharge valve is a con- 
stant in any given case, while the volume of intake air varies with 



EXPLOSIONS IN COMPRESSORS AND RECEIVERS 



179 



the speed of the compressor. Thus, a leak of 2 per cent, of the 
intake volume, at, say, 125 revolutions per minute, becomes 10 
per cent, if the compressor be slowed down to 25 revolutions. This 
agrees with experience, violent explosions being known to have oc- 
curred while the compressor was running slowly. " The oil-feed 
was probably adjusted to the maximum speed and hence was 
excessive for the slow speed. A larger proportional leak — a 
liberal quantity of oil — and the result is easily comprehended." 

Table IX 





Temperature of Discharge. Degrees Fahrenheit. 


Leakage. Per Cent. 






At Sea- Level. 


At 4,000 Feet Elevation. 





459 


496 


1 


466 


5°4 


2 


475 


5i3 


4 


489 


530 


6 


506 


549 


8 


524 


57o 


10 


544 


593 


12 


566 


618 


14 


589 


646 


16 


615 


675 



The effect of leakage of the discharge valves, moreover, is cumu- 
lative, for each rise in initial temperature thereby produced 
causes a greater rise in terminal pressure; and the leakage con- 
tinuing, a very few strokes would suffice to ignite the best cylinder 
oil. Under some circumstances, even a single stroke of the piston 
may cause ignition, if not explosion. 

The importance of minimizing piston and discharge-valve 
leakage is evident. One of the surest means of avoiding danger 
of high cylinder or receiver temperatures is the adoption of stage 
compression. There are two reasons for this: (1) the air is partly 
cooled between the stages, so that the maximum temperature is 
always less than in single-stage machines, and (2) the leakage is 
likely to be less because there is a smaller difference between the 
pressures on the two sides of the piston, as well as between the 
internal and external pressures on the discharge valves. 



l8o COMPRESSED AIR PLANT FOR MINES 

A case of explosion, in which the influence of cylinder leakage 
is not clearly apparent, occurred some years ago in the air pipe of 
a large plant in Butte, Mont. Two duplex compressors, with 
air cylinders respectively of 32 \ X 60 ins. and 24 J X 48 ins., 
and running at 50 revolutions per minute, were forcing air at 80 
lbs. pressure through a single 8-in. pipe. As somewhat over 
1,200 cu. ft. of compressed air per minute were being pro- 
duced, the velocity of flow would be nearly 3,500 ft. per min- 
ute, or 58 ft. per second. It had been noticed several times 
that a portion of the discharge pipe close to the compressor be- 
came red-hot. As no explosion took place in the compressor 
cylinders, but in the pipe only, it is probable that the oil accum- 
ulated in the pipe was vaporized and ignited. In the pipe between 
compressors and receivers there were several sharp bends, which 
increased the friction due to the rapid flow of the air. The re- 
ceivers were always extremely hot. On one occasion the shaft 
timbering, forty or fifty feet below the shaft mouth, took fire from 
the hot air pipe. The above gives point to the fact that, while the 
primary causes of explosion are to be found in the air cylinder, 
the disastrous effects are perhaps oftener observable in the dis- 
charge pipe or receiver. 

Foul or poisonous gases may result from ignition of the 
lubricant in compressors or receivers, not necessarily followed by 
actual explosion. In an article in the Trans. Anier. Inst. Min. 
Engs., Vol. XXXIV, p. 158, an instance is noted of combustion in 
the air pipe and receiver. The compressed air was being used 
in an imperfectly ventilated upraise in the mine, 1,200 ft. from 
the compressor, and 2 men lost their lives, while 4 others barely 
escaped asphyxiation. 

Other more or less similar cases are familiar to most miners, 
where foul air from the exhaust of machine drills has been ob- 
served; sometimes merely disagreeable, though often actively 
deleterious. The use of poor cylinder oil is frequently responsible 
for this, as its lighter constituents may begin to volatilize and burn 
at a perfectly normal working temperature. Even if not actually 
fried on the hot metal surfaces, a low-grade oil may yet undergo 



EXPLOSIONS IN COMPRESSORS AND RECEIVERS l8l 

a slow combustion or oxidation, which will produce enough car- 
bon dioxide to raise materially the percentage of that poisonous 
gas in the confined atmosphere of the working places of mines. 

The mode of using the lubricant for the air cylinders of com- 
pressors deserves some attention. Sight-feed lubricators, such 
as are commonly employed for steam cylinders, are best. On 
the Clifton Colliery compressor, mentioned above, ordinary 
oil-cups were used, holding about J- pint. They were filled 
4 times per day of 10 hours. With these oil-cups, if improp- 
erly adjusted, it would be possible for all the oil to be sucked 
into the cylinder within a few strokes after being filled. Such 
a result might be inferred, indeed, in this case, because of the 
large quantity of carbonaceous matter — oil, coal dust, etc. — found 
in and around the discharge valves and in the receiver. The feed- 
ing of the oil should be carefully regulated, and a smaller quantity 
used in an air cylinder than a steam cylinder of the same size — 
say, one-third as much. An excess of oil increases the tendency 
to gum the valves. For stage compressors of ordinary size, 
i drop of good cylinder oil every 4 to 5 minutes is sufficient. 

The periodical use of soap and water (soap-suds) is to be 
recommended for any compressor that cannot be shut down at 
short intervals for overhauling. It is fed into the air cylinder 
through an oil-cup, say during one day per week. Or it may be 
forced in by an oil-pump, with which the air cylinder should be 
provided. Soap and water is a poor lubricant in itself, and must 
be used more freely than oil, but it is effectual in cleansing the 
cylinder, valves, and ports from any carbonaceous or gummy 
matter that may have been deposited. If the compressor is to be 
stopped, as at the end of a shift, care must be taken to discontinue 
the feeding of soap and water some time before shutting down, 
and resume the oil-feed. This is necessary to avoid the formation 
of rust. Every compressor should be overhauled from time to 
time, and thorough cleaning should extend to all parts, especially 
around the valves and passages, capable of furnishing a lodgment 
for oil or partly oxidized carbonaceous material. 

Precautions for Preventing Explosions. These may be sum- 



1 82 COMPRESSED AIR PLANT FOR MINES 

marized as follows: (i) Always enclose the inlet valves in a cold- 
air box, connecting with the outside air, so as to avoid taking the 
air from the hot engine-room. This not only conduces to econ- 
omy in working, but by keeping down the final temperature tends 
to prevent decomposition of the oil. (2) The largest possible area 
of cylinder surface should be water-jacketed, including the cylin- 
der heads. A liberal supply of the coldest water obtainable 
should be used for the jackets. The advantages in this respect 
derived from employing stage compression, with large inter- and 
aftercoolers, are undoubted. (3) Use only the best cylinder oil, 
with high flash- and ignition-points and in as small quantity as is 
consistent with proper lubrication. Care should always be taken 
to keep the valves clean. In the design of the compressor there 
should be no recesses or pockets, around the valves or passages, 
where oil could accumulate. (4) So arrange the air intake that 
coal dust will not be drawn into the cylinder with the inlet air. 
(5) It is well to place a thermometer in the discharge pipe, 
close to the cylinder, so that the engineer will be able to note the 
temperature from time to time and stop or slow down the com- 
pressor if the temperature of the discharge air rises too high. 



CHAPTER XV 

AIR COMPRESSION BY THE DIRECT ACTION OF 

FALLING WATER 

In view of the economic importance of keeping down the tem- 
perature of the air during compression, it is evident that an ad- 
vantage would be derived from a closer and more intimate con- 
tact between the air under compression and the cooling water 
than is possible with the external water-jackets of dry compress- 
ors. From a thermodynamic standpoint it cannot be questioned 
that the wet compressor is more efficient than the dry. As has 
been shown in the latter part of Chapter V, it is mainly the me- 
chanical difficulties resulting from the use of injected water in 
the air cylinder that operate to the disadvantage of the wet system 
of compression, and that have caused its almost complete aban- 
donment. 

Since 1896 several large plants have been successfully in- 
stalled in which air is compressed by the direct action of falling 
water and without the use of piston, valves or other moving parts. 
The simple and familiar principle involved has aroused much 
interest in this method of air compression. When air in small 
bubbles is intimately mixed with water, the water breaks into 
foam, through which the air bubbles tend to rise and escape. But 
if the mixed air and water be drawn downward by a strong falling 
current, suitably confined, as in a vertical pipe, the air is com- 
pressed. And if, after reaching the depth and head of water col- 
umn necessary to produce the degree of compression desired, the 
direction of flow be changed to the horizontal and the velocity 
diminished, the air bubbles will rise. They may then be collected 
in a suitable chamber, in which the air pressure corresponds to 
the head of water and from which the air is drawn off as required. 

183 



184 COMPRESSED AIR PLANT FOR MIXES 

As the air bubbles are minute and thoroughly disseminated 
through the water during its descent, the total cooling surface 
presented is very large and complete isothermal compression re- 
sults. It should be observed also that the compressed air is 
very dry. While undergoing reduction in volume, the percen- 
tage of moisture in a given globule of air increases until the point 
of saturation is reached, but any further compression causes 
deposition of part of the moisture. Moreover, since the air is 
kept constantly cool during compression, its moisture-carrying 
capacity is smaller than if compressed adiabatically, as in an 
ordinary compressor cylinder. 

Although such an apparatus embodies no new principle, it was 
first constructed on a working scale and successfully tested, about 
1878, by J. P. Friz ell. of Boston. Mass.* Aided by this prece- 
dent, a more effective and practical method of breaking up the 
water and impregnating it with air in a state of fine division, was 
afterward devised by Charles H. Taylor, of Montreal. Canada. 
In 1896 the Taylor Hydraulic Air Compressing Co., of Mon- 
treal, erected a plant embodying the system for the Dominion 
Cotton Mills. Magog, Province of Quebec. t This plant has long 
been in successful operation, and where the conditions permit its 
introduction the system may be advantageously employed for 
mining service also. 

For the Magog Mills a 128-ft. shaft was sunk to give the 
desired head and pressure (Fig. 85). In it was erected a large 
vertical compressing pipe, a, 3 ft. 8J in. diameter, the lower part 
gradually increasing to 4 ft. 8 in., and made of i%-in. steel plate. 
This pipe passes through the bottom of an iron receiving cham- 
ber, b, at the surface, to which water is conducted from a dam 
or reservoir. The chamber, b, is 12 ft. diameter by 12 ft. high. 
"Water flows into and fills the pipe, which extends nearly to the 

* For a record of these tests see Proceedings of the Institution of Ck'ii Engineers, 
London, Vol. LXIIL p. 347. 

t The following description is based on an article in the Canadian Engineer, 
March, 1897, and information furnished to the author by the builders. See also 
Eng. and Mining Jour., Dec. 26th, 1896, p. 606, and Railway and Engineering 
Review, Sept. 17th, 1898, p. 513. 



AIR COMPRESSION BY ACTION OF FALLING WATER 185 




Fig. 85. — Taylor Hydraulic Air Compressor. 



1 86 



COMPRESSED AIR PLANT FOR MINES 



bottom of the shaft. By means of an arrangement of small feed 
pipes described below, air is drawn with the water into the top of 
the main vertical pipe and is compressed while being carried 
down the shaft. The compressed air collects in a separating 



Plan of Spider 

of Cylindrical 

Head Piece 




Fig. 86. 



chamber, c, at the bottom of the shaft, while the water is returned 
up the shaft to a tailrace near the top. The difference of water 
level between intake and tailrace is about 22 ft., which produces 
the requisite speed of flow of the mass of water. Into the top 



AIR COMPRESSION BY ACTION OF FALLING WATER 187 

of the vertical pipe, a, is inserted a telescoping section of pipe, 
d, to the upper end of which is riveted a bell-mouth, e. 
Above the latter is a cylindrical headpiece, /, 4 ft. 8 ins. diameter 
(Fig. 86), terminating below in an inverted conoid, g, projecting 
into the bell-mouth. These two parts are connected by lugs 
and bolts in such way as to leave an annular opening between 
them, through which the water enters the vertical pipe. Around 
the headpiece is set a series of thirty 2 -in. pipes, h, h, 4 ft. long, 
open at the top and closed at the bottom. Into each of these 
pipes, near their lower ends, are screwed 32 short horizontal f-in. 
pipes, i, i, all directed into the annular opening at the bell-mouth 
and toward the axis of the main pipe. As the entering water passes 
among the small pipes a tendency to vacuum is created in them, 
so that the atmospheric pressure drives the air through them into 
the water in the form of small bubbles. These are carried with 
the water down the main pipe, and on their way are compressed. 
Near the bottom of the shaft the vertical compressing pipe 
enters the large circular " separating " chamber, c, 17 ft. diam- 
eter and 12 ft. high, open below and supported upon legs which 
raise it 16 ins. above the shaft bottom. Within the tank and di- 
rectly under the pipe is the "disperser," /, a conoidal casting like 
the one in the headpiece. Plates, &, are added around the periph- 
ery of the disperser to give it an outside diameter of 12 ft. 
Below is an inverted conical apron, /, 5 ft. wide, riveted to the 
interior of the separating tank. When the water, charged 
with air bubbles, reaches the disperser it is directed outward 
toward the circumference; is then deflected by the apron tow- 
ard the center under the disperser, and finally escapes through 
the open bottom of the separating tank into the return column. 
During this process of travel the compressed air separates from 
the water, most of it collecting in the upper part of the air cham- 
ber. A portion of the air is not liberated until the water reaches 
the lower part of the tank, under the apron. This residuum 
collects in the annular space and joins the main body of air 
through the pipe, m. The compressed air collecting in the top of 
the air chamber is kept under pressure by the weight of the re- 



i88 



COMPRESSED AIR PLANT FOR MINES 



turn water column in the shaft, and is drawn off through the ver- 
tical air main, alongside of the water column a. As the small 
air bubbles are constantly surrounded by cold water, it is evi- 
dent that by this system perfect isothermal compression is at- 
tained, with its corresponding advantages in minimizing the 
amount of moisture carried off in the air. This has been shown 
by tests. 

With a total depth of shaft of 128 ft., in this installation, an 
air pressure of 52 lbs. per sq. in. is produced. The efficiency of 
this plant is shown by the following table* to be from 50.1 per 
cent, to 62.4 per cent., according to the quantity of water used: 



Table X 



No. 

of 
Test. 


Quantity 
of Water 

Dis- 
charged, 
in Cubic 
Feet per 
Minute. 


Available 

Head in 

Feet. 


Available 
Horse- 
Power. 


Quantity of Air 

Delivered, in 

Cubic Feet per 

Minute at 

Atmospheric 

Pressure. 


Pressure 
of Air, 
Pounds 

per 

Square 

Inch. 


Actual 
Horse- 
Power of 

Com- 
pressor. 


Efficiency 
of Com- 
pressor, per 
Cent. 


1 
2 

3 
4 

5 
6 


6122 

5504 
4005 
7662 
6312 
7494 


21.4 
21.9 
22.3 
21. 1 
21.7 
21.2 


247.7 
228.0 
168.9 

3°5-9 
260.0 
299.8 


1377 
1363 
i°95 
1616 
1506 
1560 


5 2 
52 
52 
52 
52 
52 


!3 2 -5 
131. 

io 5-3 
155-4 
144.8 
150.2 


53-5 

57-5 
62.4 

50.8 

55-7 
5°-i 



Temperatures during tests: external air 75 ° to 83 c 
compressed air 75.2 to 8o°. 



water 75.2 to 8o c 



The parts were not correctly proportioned in this first instal- 
lation, and there is no doubt that the efficiency could be consider- 
ably increased by using a relatively larger air chamber at the 
bottom of the shaft, to prevent air from going to waste. As shown 
by the table, the efficiency is increased by diminishing the volume 
of inlet water, upon which depends the quantity of air carried 
down and compressed. 

In building a plant to produce higher air pressure the motive 
head, or difference in level between the surfaces of wate** at inlet 
and tailrace, would be increased. The theory is as follows : The 

* Tests made by Prof. C. H. McLeod, of McGill University, August, 1896. Pub- 
lished in Eng. and Min. Journal, December 26th, 1896, d. 606. 




.=W\ ! ;M ( 


J fjj U'MMMM^ 


















1 ') 


j - 



^SM^i- 




Fig. 87.— Hydraulic Air Compressing Plani ai K ma) 



AIR COMPRESSION BY ACTION OF FALLING WATER 1 89 

combined specific gravity of the mixture of air and water in the 
vertical compressing pipe is less than that of the water in the re- 
turn column. That is, the weight of water in the compressing 
pipe is less per foot than in the return column. Therefore, the 
head required, to overcome friction and to produce flow, must be 
greater than if the apparatus were merely an inverted siphon, and 
as the difference in weight increases with depth (and air press- 
ure produced) the motive head must be correspondingly increased. 

In 1 898-1 900 a plant on the Taylor system was built for the 
Kootenay Air Supply Co., Ainsworth, British Columbia. The 
topographical conditions are such that a high head of water is 
obtained without sinking a deep shaft. From a small dam the 
water is carried in a wooden-stave pipe, 5 ft. in diameter and 
1,354 ft. long. The pipe finally passes over a short, but high 
trestle, built against the side of a steep gorge, to the receiving 
tank. The latter, 17 ft. diameter by 20 ft. high, is placed on a 
wooden tower, no ft. high (Fig. 87). From the bottom of the 
tank the pressure pipe, 33 ins. diameter, descends vertically inside 
the tower to the ground level and then down a shaft 105 ft. 
deep.* After compressing the air the water returns up the 
shaft to the tailrace at the creek level. As shown in Fig. 88, 
the details of the receiving chamber at the bottom of the shaft 
differ from those of the Magog plant. 

The effective compressing head is 107 ft., while the total 
height of the pressure pipe is over 200 ft. This produces a 
high velocity of flow and a correspondingly large delivery of 
compressed air. The main pipe line, 9 inches diameter, is 2 
miles long, discharging from 4,200 to 4,600 cu. ft. of free air 
per minute. Branch service pipes convey the air to neighboring 
mines, where it is used for rock-drills and other mining machinery. 
On the basis of 600 horse-power, represented by the volume and 
pressure of the air compressed, the cost of the entire plant, includ- 
ing pipe lines, was about $100 per horse-power. This would be 
somewhat increased by allowances for transmission and other 
losses. 

* Canadian Electrical News, September, 1898, p. 176. 



190 



COMPRESSED AIR PLANT FOR MINES 



Another large plant was completed in 1906 at the Victo- 
ria Copper Mine, near Rockland, Ontonagon Co., Michigan. 
Though the same general design was adopted for the intake head 




END ELEVATION 




SIDE ELEVATION 



^M^m^m2^^^^^m^m^i^^i^^^^^^ 




PLAN 



Fig. 88. — Hydraulic Air-Compressor at Kootenay 



AIR COMPRESSION BY ACTION OF FALLING WATER 191 

and its appurtenances, the local conditions led to a novel mode of 
installation. The water is conducted from a dam on the On- 
tonagon River through a 4,700-ft. canal, furnishing a head at 
the terminal forebay of 72 ft. above the river-level. Three 
independent units are built side by side in a vertical shaft 343 ft. 
deep. The subdivision of the air, as admitted at the intake head, 
is carried farther than in either of the plants described above, 
there being no less than 1,800 f-inch horizontal feed pipes, in- 
serted in the series of vertical pipes encircling the inverted cone. 
The compressing pipes are 5 ft. in diameter, lined with con- 
crete, and the separating cones and dispersers, also of iron and 
concrete, are built at the bottom in a rock chamber excavated for 
the purpose. In this chamber, 281 ft. long and 18 ft. X 21 ft. 
average cross-section, the compressed air is trapped and thence 
drawn off for use through a 24-inch main. The compress- 
ing water, flowing down the intake pipes, stands normally at 
a level about 14J ft. below the roof of the chamber, thus leav- 
ing an air capacity of about 80,000 cu. ft. Connected with the 
end of the air chamber is an inclined shaft, 270 ft. in vertical 
depth, through which the water returns to the surface. The tail- 
race from the mouth of this shaft is 72 ft. below the level of the 
intake, this height measuring the motive head producing the 
flow of water. Thus the air in the underground chamber is 
under a pressure due to 2^0 ft. head of water, or 117 lbs. per sq. 
in. gauge. 

For regulating the operation of the compressor a pipe passes 
from the air chamber up the compressing shaft to the surface, 
where branches from it are led to the intake heads. The com- 
pressed air conveyed in this regulating pipe operates a device con- 
nected with each intake head, whereby the latter is automatically 
raised above the water-level in the receiving tanks whenever the 
air pressure exceeds the normal, thus stopping the flow of air 
through the feed pipes. A twelve-inch blow-off pipe is also pro- 
vided, passing from the water-level in the air chamber to the mouth 
of the inclined shaft carrying the return water column. If air to 
the full compressor capacity is drawn off, the water-level in the air 



192 



COMPRESSED AIR PLANT TOR MIXES 



chamber rises as the air pressure falls, thus sealing the lower end 
of the blow-off pipe; then, when the consumption of air decreases 
the pressure in the chamber rises, depressing the water-level until 
the blow-off orifice is uncovered, when more air is blown off. 
Thus the working pressure is maintained within quite narrow 
limits. It may be added that the great size of the air chamber — 
which acts like the receiver of an ordinary air-compressor plant — 
gives it a large storage capacity. 

When all 3 compressing units are in operation, with a total 
capacity of from 34,000 to 36,000 cu. ft. of air per minute, about 
70,000 cu. ft. of free air per minute may be drawn off for a 
period of 18 minutes, without causing a drop in pressure of more 
than 5 lbs. For each unit, the output ranges from 9,000 to 12,000 
cu. ft. per minute, and the volume of water used, from 12,700 
to 14,800 cu. ft. A series of tests made on a single intake head 
in May, 1906, by Prof. F. W. Sperr, gave the following results: * 



Table XI 
Air Measurements 





Velocity. Feet Cubic Feet per 


Absolute Pressures 




Square Feet, j per Second. Minute. * 


Free Air, 
Pounds. 


Compressed 
Air, Pounds. 


Horse- Power. 


4 
4 
4 


44.09 10,580 

49-74 11,93° 
38.50 9,238 


14 
14 
14 


128 
128 

128 


1,43° 
1,623 

1,248 


Water Measurements 


Flume Area. 


Velocity. Feet Cubic Feet per Ufi i -c-^.,. 
per Second. • Minute. Weaci, .beet. 


Horse-Power. 


Efficiency, per 
Cent. 


7 1 -75 

67.03 

72. 16 


3-033 x 3,°57 7o-5 
3.684 14,820 70.0 
2.936 12,710 70.6 


i,74i 
1,961 
1,700 


82.17 
82.27 
73-5° 



The air is used at the Victoria Mine for general power pur- 
poses at the mine and mill, including a 500-horse-power hoisting 

* For further details see article by D. E. Woodbridge, Engineering and Mining 
Journal, Jan. 19th, 1907, p. 125. Also, A. H. Rose, Mines and Minerals, March, 
I9°7, P- 346. 



AIR COMPRESSION BY ACTION OF FALLING WATER 1 93 

engine, designed for a depth of 4,000 ft., 7 pumps, and many other 
engines. The cost per horse-power is only about $2.25 per 
year, including all the operating expenses. It is expected that 
over 4,000 horse-power will be developed when all 3 compressing 
units are in operation. The present stage of the development of 
the mine requires the use of but 1 unit. 

The compression of air by direct action of falling water, 
according to the Taylor system, has been adopted in several other 
recent installations : two in Germany and a very large plant for 
general power purposes, on the Shetucket River, near Norwich, 
Conn.* It is probable that the application of the system will be 
extended in regions where large water powers can be developed. 
Its first cost is not excessive, while the maintenance and running 
expenses are extremely low, as compared with those of the usual 
forms of air compressors. No skilled attendance is required, and 
the item of depreciation is merely nominal in such substantially 
erected plants as that at the Victoria Mine. By comparing the 
figures given in Tables X and XI, it will be seen that in the later 
installation a very marked increase was made in efficiency of 
operation ; due to improved design of the intake head, increase in 
motive head producing the flow of the compressing water, and a 
more complete separation of the air from the water in the receiv- 
ing chamber. 

It has been suggested that it might be feasible to employ the 
system in connection with an ordinary compressor plant. That is, 
to produce a low air pressure by the water plant, and then to ad- 
mit this air to the compressor cylinder where it would be brought 
up to the required higher tension. In effect, this would be stage 
compression, in which the air would be completely cooled to nor- 
mal temperature before entering the high-pressure cylinder. 

* The last-mentioned is described in Compressed Air, April, 1906, p. 3,980. 



13 



Part Second 



TRANSMISSION AND USE OF 
COMPRESSED AIR 



CHAPTER XVI 

CONVEYANCE OF COMPRESSED AIR IN PIPES 

Certain losses due to friction take place in conveying com- 
pressed air through lines of piping. The diameter of the pipe is 
of vital importance, and when proportioned properly to the 
volume of air, and to the distance, these transmission losses 
are very small as compared with the other losses incident upon air 
compression. With the possible exception of electricity, no other 
means of power transmission can compare in efficiency with 
compressed air. The transmission losses appear in two ways: 
as loss of power, and as loss of pressure or head, indicated by 
difference in gauge reading at the ends of the line. Between these 
two losses there is a clear distinction. 

Loss of Power. The large and, to a great extent, unavoidable 
loss of power due to the heating of the air during compression 
and its subsequent cooling after leaving the compressor, has 
already been considered. But this cooling takes place so quickly 
in the receiver and piping that the resulting loss is not properly 
chargeable to transmission. The air assumes the temperature of 
the surrounding atmosphere in the first few hundred feet, so that 
when conveyed to long distances the calculation for transmission 
loss may be made without regard to the effect of temperature upon 
the volume of the air. In other words, the volume is taken simply 
as proportional to the absolute temperature, in atmospheres. 

194 



CONVEYANCE OF COMPRESSED AIR IN PIPES 1 95 

The power residing in the compressed air is due not only to its 
pressure, but also to its volume, in terms of number of cubic feet 
of free air (i.e., air at atmospheric pressure). Thus, while the 
pressure is reduced by frictional loss in transmission, yet this 
reduction in pressure is accompanied by a proportionate increase 
in volume, and a certain compensation is produced. Although the 
pressure of the air at the motor is diminished, there is no loss in 
the final volume of free air. As will be shown below, the loss of 
pressure due to the conveyance of air in pipes is small, but the 
actual loss of power is still smaller. The pipe itself acts in a 
measure like a receiver — as a reservoir of power. It is probable 
that much of the transmission power loss experienced in practice 
is due to leakage from joints and flaws in the pipe. 

Loss of Pressure or Head. For short distances the loss of 
pressure may be considered as taking place according to the laws 
governing the flow of all fluids, varying directly as the length of 
pipe, directly as the square of the velocity, and inversely as the 
diameter of the pipe. But for long distances the application of 
these laws becomes somewhat complex. In addition to the fac- 
tors just given, it is necessary to take into account the volume 
and pressure of the air, and the difference between the pressures 
at the receiver and at the end of the pipe line. All are more or 
less interdependent. A statement of the case, more accurate 
than the above, is as follows : For a given diameter of pipe, when 
the volume of compressed air discharged and its initial pressure 
remain constant, the loss of pressure is proportionate to the 
length of the pipe. 

But in actual service the initial pressure and the volume of 
discharge do not remain constant, and, in the passage of the air 
through the pipe, other modifying factors must be taken into 
account. In flowing through a long line of piping the pressure is 
gradually reduced by friction, while the volume is correspondingly 
increased. Therefore, to maintain in the pipe the flow of a 
given quantity of air whose volume is constantly increasing, the 
velocity also must increase, and this requires an increase of head 
or pressure. 



196 COMPRESSED AIR PLANT FOR MIXES 

The formulas commonly used are constructed on the hy- 
pothesis that the loss of head is proportional to the length of pipe, 
so that, if a certain head be required to maintain the flow of a 
given quantity of air in a pipe 1,000 feet long, twice this head 
would suffice for a pipe 2,000 feet long. But in this case, when 
the air has passed through the first thousand feet of pipe its mo- 
tive head has been lost; and as the volume has thereby increased, 
a greater head will be necessary to maintain the flow in the second 
thousand feet. In other words, the ordinary formulas do not 
take into account the increase of volume due to the reduction 
of pressure, i.e., loss of head. 

To transmit a given volume of air at a uniform velocity and 
loss of pressure it would be necessary to construct the pipe with a 
gradually increasing area. This of course is impracticable, and 
if the rate of discharge is to be kept constant in pipe of uniform 
section, both volume and velocity must increase as the pressure is 
reduced by friction. The loss of head in properly proportioned 
pipes is so small, however, that in practice the increase in volume 
is usually neglected. 

The actual discharge capacity of piping is not proportional 
to the cross-sectional area alone — that is, to the square of the 
diameter. Although the periphery is directly proportional to the 
diameter, the interior surface resistance is much greater in a small 
than in a large pipe, because as the pipe becomes smaller the 
ratio of perimeter to area increases. To pass a given volume 
of compressed air a i-in. pipe of given length requires over 
3 times as much head as a 2-in. pipe of the same length. The 
character of the pipe also, and the condition of its inner sur- 
face, have much to do with the friction developed by the flow 
of air. Besides imperfections in the surface of the metal, the 
irregularities incident upon coupling together the lengths of pipe 
must increase friction. 

There are so few reliable data that the influences by which 
the values of some of the factors may be modified are not fully 
understood, and owing to these uncertain conditions the results 
obtained fromformulas are only approximately correct. Among 



CONVEYANCE OF COMPRESSED AIR IN PIPES 



197 



the formulas in common use for determining the loss of pressure 
in pipes perhaps the most satisfactory is that of D'Arcy. As 
adapted for compressed-air transmission it takes the form: 

:\/d* 



■H^^-w*^ 



-A 



w* 



in which 

D=the volume of compressed air in cubic feet per minute 

discharged at the final pressure, 
c = a coefficient varying with the diameter of the pipe, as de- 
termined by experiment, 
d = diameter of pipe in inches,* 
/ = length of pipe in feet, 

p t = initial gauge pressure in pounds per square inch, 
p 2 = final gauge pressure in pounds per square inch, 
w x = the density of the air, or its weight in pounds per cubic 

foot, at the initial pressure p x . 
The second form of the formula, as given above, will be found 
convenient for most calculations, as the factors can be considered 
in groups. 

In the following table are given the values of c, d 5 , and c Vd'\ 
The values of c show some apparent discrepancy for sizes of pipe 

Table XII 



Diameter of Pipe, 


Values of 


Fifth Powers of 


Values of 


Inches. 


c 


d 


c\/d* 


1 


45-3 


1 


45-3 


2 


52.6 


32 


297 


3 


56-5 


243 


876 


4 


58.0 


1024 


1856 


5 


59-° 


3 I2 5 


3298 


6 


59-8 


7776 


5273 


7 


60.3 


16807 


7817 


8 


60.7 


32768 


10988 


9 


61.0 


59°49 


, 14812 
^9480 


10 


61.2 


1 00000 


11 


61.8 


161051 


24800 


12 


62.0 


248832 


30926 



* The actual diameters of wrought -iron pipe are not the same as the nominal 
diameters for all sizes. This difference is small, however, except in the i^-in. and 
i^-in. sizes, the actual diameters of which are 1.38 ins. and 1.61 ins. respectively. 



198 



COMPRESSED AIR PLANT FOR MINES 



larger than nine inches, but there would be no very material 
differences in the results. 

Table XIII gives the values of w x for initial gauge pressures up 
to 100 pounds per square inch : 



Table XIII 



Gauge Press- 
ure, Pounds. 


w 1 


_ 


Gauge Press- 
ure, Pounds. 


w 1 


/y/tf, 





0.0761 


0.276 


55 


0.3607 


O.60O 


5 


0. 1020 


0.3T9 


60 


0.3866 


O.622 


10 


0.1278 


0.358 


65 


0.4125 


O.642 


1 5 


°-!537 


0.392 


7° 


0-4383 


O.662 


20 


0.1796 


0.424 


75 


0.4642 


O.681 


2 5 


0.2055 


o-453 


80 


0.4901 


O.70O 


3° 


0.2313 


0.481 


85 


. 5 1 60 


O.718 


35 


0.2572 


0.507 


90 


0.5418 


O.736 


40 


0.2831 


°-53 2 


95 


0.5677 


0-753 


45 


0.3090 


°-55 6 


100 


o-593 6 


O.770 


5° 


0.3348 


o.578 









To facilitate computations in connection with D'Arcy's 
formula, Table XIV has been compiled by Mr. William 

Cox. It gives the values of A I 2± — o for terminal gauge pressures 

\ w t 

of from 20 to 100 lbs., and for pressure losses of from 1 to 10 lbs.* 
Intermediate values can be obtained by interpolation. No 

allowance is made for pipe leakage, nor for incidental friction due 

to bends in the pipe. 

By using these tables all ordinary problems involved in 

compressed-air transmission can be readily solved. For example, 

given a 5-in. pipe, 2,500 ft. long; how many cubic feet of air per 

minute at an initial pressure of 70 lbs. can be transmitted, with 

a loss of pressure of not more than 3 lbs. ? 

From Table XII, c\ZdF= 3,298; from Table XIV,. \ tl 



\ 



it\ 



= 2.570 and v7 = 50- Substituting in the formula already given : 



D 



3> 2 9 8 
5o 



X 2.570 = 169.5 cu - ft. compressed air per minute. 



* Reproduced by permission from Compressed Air, Feb., 1898, pp. 374-376. 



CONVEYANCE OF COMPRESSED AIR IN PIPES 



199 



Table XIV 



Values of 



P1-P2 



Final 


Losses of Pressure, pi—p 2 . 


Press- 




ure 






















Pi, lbs. 


ilb. 


2 lbs. 


3 lbs. 


4 lbs. 


5 lbs. 


6 lbs. 


7 lbs. 


8 lbs. 


9 lbs. 


10 lbs. 


20 


2-325 


3-241 


3-918 


4.466 


4-93° 


5-336 


5-693 


6.014 


6.309 


6-574 


21 


2.293 


3.198 


3.868 


4- 


410 


4- 


870 


5- 


272 


5- 


627 


5- 


946 


6. 


237 


6.502 


22 


2.262 


3-157 


3-819 


4- 


356 


4- 


812 


5- 


211 


5- 


564 


5- 


878 


6. 


168 


6.432 


23 


2-233 


3- JI 7 


3-772 


4 


3°4 


4 


756 


5- 


152 


5- 


501 


5- 


814 


6. 


102 


6.362 


24 


2.205 


3-°79 


3-727 


4 


254 


4- 


702 


5- 


o93 


5- 


440 


5- 


752 


6. 


036 


6.296 


25 


2.178 


3.042 


3.684 


4 


206 


4 


649 


5- 


036 


5- 


381 


5- 


688 


5- 


973 


6-233 


26 


2.152 


3.007 


3.642 


4- 


158 


4 


597 


4 


981 


5- 


323 


5- 


630 


5- 


913 


6.173 


27 


2.127 


2-973 


3.601 


4- 


112 


4 


548 


4- 


928 


5- 


268 


5- 


572 


5- 


856 


6-113 


28 


2.103 


2-939 


3-56i 


4- 


068 


4- 


499 


4- 


877 


5- 


215 


5- 


5i8 


5- 


799 


6.056 


29 


2.079 


2.907 


3-523 


4 


024 


4 


452 


4- 


828 


5- 


164 


5- 


466 


5- 


745 


5-999 


3° 


2.056 


2.876 


3-485 


3- 


982 


4 


408 


4 


781 


5- 


114 


5- 


414 


5- 


691 


5-942 


3 1 


2.034 


2.844 


3-448 


3 


942 


4 


365 


4 


735 


5- 


066 


5- 


364 


5- 


637 


5.888 


32 


2.012 


2-815 


3-414 


3- 


904 


4 


323 


4- 


690 


5- 


019 


5- 


312 


5- 


586 


5-834 


33 


1. 991 


2.786 


3-38i 


3 


866 


4- 


282 


4 


646 


4- 


971 


5- 


264 


5- 


535 


5-782 


34 


1. 971 


2-759 


3-348 


3- 


830 


4 


242 


4 


603 


4- 


926 


5- 


216 


5- 


487 


5-733 


35 


1-952 


2-733 


3-3 T 7 


3 


794 


4- 


202 


4- 


561 


4 


881 


5- 


170 


5- 


439 


5.686 


36 


1-933 


2.707 


3.286 


3 


758 


4- 


164 


4 


520 


4- 


839 


5- 


126 


5- 


394 


5-639 


37 


i-9i5 


2.682 


3-255 


3 


724 


4 


126 


4- 


480 


4- 


797 


5- 


084 


5- 


349 


5-594 


38 


1.897 


2.656 


3- 2 25 


3 


690 


4 


090 


4- 


441 


4 


757 


5 


042 


5 


307 


5-55° 


39 


1.879 


2.632 


3.196 


3 


658 


4 


054 


4 


404 


4- 


717 


5 


002 


5 


265 


5-509 


40 


1.862 


2.608 


3.168 


3 


626 


4 


020 


4 


368 


4 


680 


4. 


962 


5 


226 


5-468 


41 


1.845 


2-585 


3.140 


3 


596 


3 


987 


4 


333 


4 


643 


4 


924 


5- 


187 


5.426 


42 


1.829 


2-563 


3- JI 4 


3 


566 


3 


956 


4 


299 


4 


609 


4 


888 


5 


148 


•5-385 


43 


1. 813 


2.542 


3.088 


3 


538 


3 


924 


4 


267 


4 


575 


4 


852 


5 


109 


5-344 


44 


1.798 


2.521 


3.064 


3 


5io 


3 


■895 


4 


235 


4 


•54o 


4 


814 


5 


070 


5-3o6 


45 


1-783 


2.501 


3.040 


3 


.484 


3 


.866 


4 


.203 


4 


.506 


4 


•778 


5 


•034 


5.268 


46 


1.769 


2.481 


3.017 


3 


•458 


3 


•837 


4 


.171 


4 


.471 


4 


■744 


4 


•998 


5-230 


47 


i-755 


2.462 


2-995 


3 


■432 


3 


.808 


4 


• J 39 


4 


■439 


4 


.710 


4 


.962 


5.192 


48 


1.742 


2.444 


2.972 


3 


.406 


3 


•779 


4 


.109 


4 


.408 


4 


.676 


4 


.926 


5-155 


49 


1.729 


2.426 


2.950 


3 


•380 


3 


■752 


4 


.080 


4 


•376 


4 


.642 


4 


.890 


5.120 


50 


1. 716 


2.407 


2.927 


3 


.356 


3 


■725 


4 


•051 


4 


■344 


4 


.608 


4 


.857 


5-085 


5i 


1-703 


2-389 


2.906 


3 


■332 


3 


.698 


4 


.022 


4 


■3*3 


4 


•578 


4 


.824 


5-050 


5 2 


1.690 


2.372 


2.886 


3 


.308 


3 


.671 


3 


•993 


4 


•283 


4 


■546 


4 


.791 


'5-oi5 


53 


1.678 


2-355 


2-865 


3 


.284 


3 


•645 


3 


■965 


4 


.254 


4 


.516 


4 


•758 


4-983 


54 


1.666 


2-338 


2.844 


3 


.260 


3 


.620 


3 


•938 


4 


.225 


4 


.484 


4 


.728 


4-952 


55 


1.654 


2.321 


2.823 


3 


.238 


3 


■596 


3 


.Oil 


4 


.196 


4 


■456 


4 


.698 


4.920 


56 


1.642 


2.304 


2.804 


3 


.216 


3 


■57i 


3 


.885 


4 


.169 


4 


.428 


4 


.668 


4.889 


57 


1.630 


2.289 


2-785 


3 


.194 


3 


•547 


3 


860 


4 


■143 


4 


.400 


4 


.638 


4.860 


58 


1. 619 


2.273 


2.766 


3 


.172 


3 


•5 2 4 


3 


835 


4 


117 


4 


■372 


4 


.611 


4.832 


59 


1.608 


2.258 


2-747 


3 


.152 


3 


502 


3 


811 


4 


091 


4 


•346 


4 


.584 


4-803 


60 


1-597 


2.242 


2.730 


3 


.132 


3 


479 


3 


787 


4 


066 


4 


.320 


4 


•557 


4-775 


61 


1.586 


2.228 


2.712 


3 


.112 


3 


458 


3 


764 


4 


042 


4 


294 


4 


53o 


4-747 


62 


i-576 


2.214 


2.695 


3 


.092 


3 


437 


3 


742 


4- 


019 


4 


268 


4 


5°3 


4.718 


63 


1.566 


2.200 


2.678 


3 


.074 


3 


4i7 


3 


720 


3 


995 


4 


244 


4 


476 


4-693 


64 


i-556 


2.186 


2.662 


3 


.056 


3 


397 


3- 


698 


3- 


971 


4 


220 


4 


452 


4.668 


65 


1.546 


2.173 


2.647 


3 


.038 


3 


376 


3- 


676 


3- 


948 


4 


196', 4 


428 


4.642 


66 


i-537 


2.160 


2-631 


3 


.020 


3 


356 


3- 


654 


3- 


926 


4 


172 


4 


404 


4.617 



200 



COMPRESSED AIR PLANT FOR MINES 



Table XIV — Continued 



Wy 



Final 


Losses of Pressure, Pi—p 2 . 


Press- 




ure. 




















Pi, lbs. 


ilb. 


2 lbs. 


3 lbs. 


4 lbs. 


5 lbs. 


6 lbs. 


7 lbs. 


8 lbs. 


9 lbs. 


10 lbs. 


67 


1.528 


2.147 


2.615 


3.002 


3-337 


3-634 


3-905 


4.150 


4-38o 


4-592 


68 


I-5I9 


2.134 


2.600 


2.984 


3-3i8 


3-6i5 


3.884 


4.128 


4-356 


4 


.566 


69 


1. 510 


2.122 


2.584 


2.968 


3-3°° 


3-596 


3-863 


4.104 


4-332 


4 


-541 


70 


1. 501 


2.100 


2.570 


2.952 


3-283 


3-576 


3.842 


4.082 


4-3o8 


4 


.516 


7i 


1.492 


2.098 


2-556 


2.936 


3-265 


3-556 


3.820 


4.060 


4.284 


4 


•494 


72 


1.484 


2.086 


2-543 


2.920 


3- 2 47 


3-537 


3-799 


4-038 


4.263 


4 


.471 


73 


1.476 


2.075 


2.529 


2.904 


3.229 


3-517 


3-778 


4.018 


4.242 


4 


•449 


74 


1.468 


2.064 


2-515 


2.888 


3 . 2 1 1 


3-498 


3-759 


3-998 


4.221 


4 


•427 


75 


1.460 


2.052 


2.501 


2.872 


3-J93 


3.480 


3-74i 


3-978 


4.200 


4 


•405 


76 


1.452 


2.041 


2.487 


2.856 


3-177 


3-463 


3-723 


3-958 


4.179 


4 


■383 


77 


1.444 


2.030 


2-473 


2.842 


3.162 


3-446 


3-704 


3-938 


4.158 


4 


.361 


78 


1.436 


2.019 


2.461 


2.828 


3.146 


3-429 


3.686 


3.918 


4-137 


4 


339 


79 


1.428 


2.009 


2.449 


2.814 


3-*3° 


3.412 


3.667 


3-898 


4. 116 


4 


3i7 


80 


1. 42 1 


1.999 


2-437 


2.800 


3-"5 


3-395 


3-648 


3.878 


4-095 


4 


294 


81 


1. 414 


1.989 


2.425 


2.786 


3-oo9 


3-377 


3-630 


3-858 


4.074 


4 


272 


82 


1.407 


1.979 


2-413 


2.772 


3.084 


3-360 


3-611 


3-840 


4-053 


4 


253 


83 


1.400 


1.969 


2.401 


2-758 


3.068 


3-343 


3-593 


3.820 


4-035 


4 


234 


84 


1-393 


1-959 


2.388 


2-744 


3-052 


3-326 


3-575 


3.802 


4-OI7 


4- 


215 


85 


1.386 


1.949 


2.376 


2.730 


3-037 


3-3 10 


3-559 


3-786 


3-999 


4- 


196 


86 


1-379 


!-939 


2.364 


2.716 


3.022 


3-294 


3-543 


3-768 


3-98i 


4- 


177 


87 


1.372 


1.929 


2-352 


2.702 


3.008 


3-279 


3-527 


3-752 


3-963 


4- 


158 


88 


1-365 


1.920 


2.340 


2.690 


2-994 


3-265 


3-5 11 


3-734 


3-945 


4- 


135 


89 


1-358 


1. 910 


2.330 


2.678 


2.981 


3- 2 5o 


3-495 


3-7i8 


3-927 


4- 


120 


90 


J^ 1 


1. 901 


2.319 


2.666 


2.967 


3- 2 35 


3-479 


3.700 


3-909 


4- 


101 


9i 


i-345 


1.893 


2.309 


2.654 


2-954 


3-221 


3-463 


3-684 


3.891 


4- 


082 


92 


1-339 


1.884 


2.298 


2.642 


2.940 


3.206 


3-447 


3.666 


3-873 


4- 


064 


93 


!-333 


1.876 


2.288 


2.630 


2.927 


3-!9i 


3-432 


3-650 


3-855 


4- 


048 


94 


!-3 2 7 


1.867 


2.278 


2.618 


2.914 


3-J77 


3.416 


3-634 


3-840 


4- 


032 


95 


1. 321 


1-859 


2.267 


2.606 


2.900 


3.162 


3.401 


3-618 


3-825 


4- 


016 


96 


I -3 I 5 


1.850 


2.257 


2-594 


2.887 


3.148 


3-387 


3-604 


3-8io 


4- 


000 


97 


1.309 


1.842 


2.246 


2.582 


2.873 


3-*35 


3-373 


3-59o 


3-795 


3- 


984 


98 


1-303 


1-833 


2.236 


2.57o 


2.862 


3- 12 3 


3-360 


3-576 


3-78o 


3- 


969 


99 


1.297 


1.825 


2.226 


2.560 


2.851 


3. no 


3-347 


3-562 


3-765 


3- 


953 


100 


1. 291 


1. 817 


2.217 


2.550 


2.840 


3-098 


3-334 


3-548 


3-75o 


3-937 



Volumes of compressed air are easily converted into corre- 
sponding volumes of free air by multiplying by the absolute press- 
ure in terms of atmospheres (1 atmosphere = 14.7 lbs.). Thus, 
100 cu. ft. of air at 80 lbs. gauge pressure, or 94.7 absolute 
pressure, are equal to 644 cu. ft. of free air, at sea-level. 
Table VIII gives the air pressures in pounds per square inch for 



CONVEYANCE OF COMPRESSED AIR IN PIPES 201 

altitudes up to 15,000 ft, with the corresponding barometric 
readings. 

Another formula for the loss of pressure in pipes has been 
published by Mr. Frank Richards, as follows : * 

V 2 L 
10,000 Da 
D= diameter of pipe in inches. 
L = length of pipe in feet. 
V = volume of compressed air delivered, in cubic feet per 

minute. 
H = head or difference of pressure required to overcome fric- 
tion and maintain the flow. 
a = constant for diameter of pipe. 

Values of a for Different Nominal Diameters 
Of Wrought-Iron Pipe. 



I" . 


. . 0.350 


3" ■ 


• • 0.730 


5 • 


. . 0.934 


Ii". 


. . o.5oof 


3i"- 


. . 0.787 


6" . 


. 1. 000 


ir- 


. o.662f 


4" - 


. 0.840 


8" . 


. . 1. 125 


2" . 


. . 0.565 






10" . 


. 1.200 


*r. 


. . 0.650 






12" . 


. 1 . 260 



Using this formula with its constants, the calculated losses of 
pressure are smaller, and, conversely, the volumes of air discharged 
are larger, under the same conditions, than those obtained from 
D'Arcy's formula. 

The losses of pressure in a table by F. A. Halsey indicate that 
the constants used by him differ materially from those given 
above. For comparison a series of random examples are shown 
in Table XV. 

An examination of this table shows that in all cases the figures 
from D'Arcy's formula lie between the others, and until further 
experimental data are available it would appear safe to conclude 
that the results obtained from this formula are sufficiently ac- 

* American Machinist, Dec. 27th, 1894. 

t The values of a for 1^- and ij-in. pipe are not consistent with those for 
other sizes. See foot-note on page 197. 



202 



COMPRESSED AIR PLANT TOR MIXES 



Table XV 



Cubic Feet of 
Air at Final 


Length of Pipe. 
Feet. 


Diameter of 
Pipe. Inches. 


Transmission losses, P 


OUNDS. 


Pressure. 


Richards. 


D'Arcy 1 Cox). 


Halsey. 


1,000 


I. coo 


4 


3- 2 3 


3-71 


q.02 


I.OOO 
I.OOO 

4.000 
4.000 


1.000 
1,000 

5. ceo 

5,000 


5 

6 

8 

10 


-95 
■35 

5.92 

1.78 


1. 17 

-46 

8-44 

2.81 


I.63 

.64 

4.20 


4,000 


5.000 


12 


.68 


1.06 


1.70 



curate for ordinary calculations. It must be remembered that, 
within certain limits, the loss of head or pressure increases with 
the square of the velocity. To obtain the best results it is found 
in practice that the velocity of flow in the main air pipes should 
not exceed twenty or twenty-five feet per second. Experiments 
made to determine the loss of pressure in the mains of the Paris 
compressed-air plant gave the following results : * 

Table XVI 
Dlaaieter of Pipe, Twelve Inches 



Velocity of Flow in 
Feet per Second. 



Initial Pressure, 
Pounds. 



Final Pressure, 
Pounds. 



Per Cent, of Initial 
Pressure Lost per Mile. 



*!> 



100 
100 

100 



97-6 
90.6 

53-8 



2.4 

9-4 

46.2 



It is evident that when the initial velocity much exceeds 50 
ft. per second the percentage loss becomes very large; and, fur- 
thermore, by using piping large enough to keep down the velocitv 
the friction loss may be almost eliminated. For example, at the 
Hoosac tunnel, in transmitting 875 cu. ft. of free air per 
minute at an initial pressure of 60 lbs., through an 8-in. pipe 
7,150 ft. long, the average loss including leakage was only 2 lbs. 
The velocity in this case was 8J ft. per second. A volume of 
500 cu. ft. of free air per minute, at 75 lbs. gauge pressure, can 

* L'nwin. Van Xostrand's Science Series, Xo. 106, p. 78. 



CONVEYANCE OF COMPRESSED AIR IN PIPES 203 

be transmitted through 1,000 ft. of 3-in. pipe with a loss of 4.1 
lbs., while if a 5-in. pipe were used the loss would be reduced to 
.24 lb., the velocities being respectively 28 ft. and 10 feet per sec- 
ond. In driving the Jeddo mining tunnel, at Ebervale, Luzerne 
Co., Penna., two 3^-in. machine drills were used in each heading, 
with a 6-in. main, the maximum distance of transmission being 
about 10,800 ft. This pipe was so large in proportion to the vol- 
ume of air required for the 2 drills (about 230 cu. ft. free air per 
minute) that the loss was reduced to an extremely small quan- 
tity, the velocity being only 3 J ft. per second. A calculation 
shows a loss of .002 lb., and the gauges at each end of the 
main were found to record practically the same pressure. 

A due regard for economy in installation, however, must limit 
the use of very large piping, the cost of which should be considered 
in relation to the cost of air compression in any given case. Diam- 
eters of from 4 to 6 ins. for the air mains are large enough for 
operating simultaneously from 6 to 10 drills. Up to a length 
of 3,000 ft. a 4-in. pipe will carry per minute 480 cu. ft. of free 
air compressed to 82 lbs., with a loss of 2 lbs. pressure. This vol- 
ume of air will run four 3-in. drills. Under the same conditions 
a 6-in. pipe, 5,000 ft. long, will carry 1,100 cu. ft. of free air per 
minute, or enough for 10 drills in constant operation. 

A mistake is often made in putting in branch pipes of too 
small a diameter. For a distance of, say, 100 ft. a 1 J-in. pipe is 
small enough for a single drill, though i-in. is frequently used. 
While it is, of course, admissible to increase the velocity of 
flow in short branches considerably beyond 20 ft. per second, 
extremes should be avoided. To run a 3-in. drill from a i-in. 
pipe 100 ft. long would require a velocity of flow of about 55 ft. 
per second, causing a loss of 10 lbs. pressure. In this connection 
Table XVII* may be studied with advantage. 

Compressed-Air Piping. The pipe for conveying compressed 
air may be of cast or wrought iron. If of wrought iron, as is 
customary, the lengths are connected either by sleeve couplings 
or by cast-iron flanges into which the ends of the pipe are ex- 

* From the catalogue of the Norwalk Iron Works Co. 



204 



COMPRESSED AIR PLANT FOR MIXES 

Table XVII 



Nomina 
of Pipe, i 


Size _ 


in. 1 J 


in. 1 h in. 2 ms. 2 J ins. 




Length of Pipe 
in Feet. [^^ 


50 


100 100 


300 j 100 


300 


200 500 250 


600 


. in 

fa 

fa 
-r. 

X 

fa 
2^ 




> 

fa 
fa 

> 

i— i 

fa 

n 

fa 


Z 

o 

- 

fa 

X 
H 

H 

< 
x. 

fa 

fa 
P 

m 
tn 
fa 

- 


79-8 


23.2 


16.4 35-2 


20.3 63.6 


36-7 


84-7 


53.6142. 91.7 




79-6 


33- 1 


23.4 49-7 


28.7 89.9 


51.9 


119. 6 


75.7 200.9 129.6 




79-4 

... 


40.4 


28.6 61.0 


35.2 109. 1 


63.0 


146.5 


92.5 


244.4 
283.2 


I 57-7 


M 


79- 2 


46.8 


33- 1 7°-3 


4O.6 I27. T 


73-4 


169. 1 


107.1 


183. 1 


fa 
'—i 




79- 


52.3 


37.0 78.6 


45-4 142.0 


82.0 189. 1 


119. 7317. 1 204.6 


< 

x. 




■y. 


7 8 - 8 57-i 


40 . 4 86.1 


49-7 155-4 


89.7207. 


131. 348.4 224.8 


~ 


►J 

c 


78.6 61.6 


43-6 93-° 


53-7 168.0 


97.0223.3 


141-3 377-o 


243-9 


— 

< 

2^ 


00 


78.4 j 65.9 


46.6 99.2 


57-3 179-3 


103.5 238-7 


151. 1 399.6258.4 


— 
fa 


78-2; 7 o. 3 


49-7 io 5--» 


60.8 190.5 


110.0 252.9'i6o.i 424.1 273.6 


< 

— 




7 8 - 73-7 


52. 1 no. 8 


64.0 200.7 IJ 5-9 266.5 168.7 446-7 288.6 


fa 

X. 
X. 

- 

- 


fa 

C 


77-8 77 . 2 


54.6 116. 2 


67.1 209.9 121. 2 279.2 176.7 469.0302.6 


w 
o 


77-6 80.7 


57-i 121. 4 


70.1 219. 1 126.5 291-5 l8 4-5 489-6315.9 


U 


< 

H 

w 


77-4 84.0 


59.4126.3 


72.9 228.1131.7 


303.4 192.0 509. 3328. 6 


X. 

< 


77-2 87.1 


61.6 131. 1 


75.7236.7 136.6 


314.4 199.0528.3 


34o.8 


- 


= 

H 

H 

<■ 


77- 


90-3 


63-7 135-4 


78.2 245.2 141. 6 


325.5 206.0546.5 


35 2 -6 


fa 
> 


76.8 


92-9 


65-7 139-8 


8o.7 : 252.4 145.7 


336.1 212.7564.2 


364-0 


fa 
fa 




w 


76.6 

' 1 


95-6 


67-7 143-9 


83.1 259.8 150.0346.2 219. 1 581.3 


375-o 


fa 
M 

< 


fa 
c/3 


76.4 


98-4 


69.6 148. 1 


85.5 267.6 154.5 


356.o 


22 5-3 597-5 


385-5 


fa 
m 


W 

tin 


76.2 IOI.O 


71.5 152.1 


87. S 274.7 158.7 


365-6 


231. 4613. 8 


396-o 




S 
tf 


76. 103.8 


73-4 156-1 


90.1 281.3 J62.4 


375-6237.31629.3 


406.0 




o 
fa 

z 

p 


75-8 106.3 


75-2 i59-7 ; 


92.2 288.4 166.6 


383.9243.0.644.5 


415.8 


fa 
fa 

fa 


75-6 


108.7 


76-9 163.3 


94.3295.5 


170.6 


392.8 248.6 659.2 


425-3 



*•* 




75-4 


III.O 


78.5 


167.0 


96.4301.7 174.2 


401.4 254.0 


673-8 


434-7 


p 
U 




75-2 


11 3'3 


80.1 


170.4 


98.4307.9 177.8 


409-7259-3 


687.8 


443-8 






"5-5 


81.7J173.9 


100.4 3 x 4-3 J 8i-5 417-9 264.5 7 OI - 6 45 2 -7 





CONVEYANCE OF COMPRESSED AIR IN PIPES 205 

panded or screwed. Sleeve couplings are used for all except the 
large sizes. The smaller sizes, up to ij in. are butt-welded, 
while all from ij in. up are lap-welded to insure the necessary 
strength. Extra heavy piping may be had for higher pressures 
than those commonly used. Wrought-iron spiral-seam riveted, 
or spiral-weld steel, tubing is sometimes used. It is made in 
lengths of 20 ft. or less. For convenience of transport in re- 
mote regions rolled sheets in short lengths may be had. They 
are punched around the edges, ready for riveting, and are packed 
closely, 4, 6 or more sheets in a bundle. 

All joints in air mains and branches should be carefully made. 
The pipe may be tested from time to time by allowing the air at 
full pressure to remain in the pipe long enough to observe the 
gauge. In case a leak is indicated it should be traced and stopped 
immediately. Air leaks are more expensive than steam leaks 
because of the losses already suffered in compressing the air. In 
putting together screw joints care should be taken that none of 
the white lead or other cementing material is forced into the pipe. 
This would cause obstruction and increase the friction loss. Also, 
each length as put in place should be cleaned thoroughly of all 
foreign substances which may have lodged inside. To render 
the piping readily accessible for inspection and stoppage of leaks 
it should, if buried, be carried in boxes sunk just below the sur- 
face of the ground; or, if underground, it should be supported 
upon brackets along the side of the mine workings. Low points 
in pipe lines, which would form" pockets " for the accumulation of 
entrained water, should be avoided, as they obstruct the passage 
of the air. In long pipe lines, where a uniform grade is im- 
practicable, provision may be made near the end for blowing out 
the water at intervals, when the air is to be used for pumps, 
hoists, or other stationary engines. 

For long lengths of piping expansion joints are required, par- 
ticularly when on the surface. Underground they are not often 
necessary, as the temperature is usually nearly constant, except in 
shafts, or elsewhere, where there may be considerable variations 
of temperature between summer and winter. 



206 



COMPRESSED AIR PLANT FOR MINES 



As each bend or elbow in a pipe line has a serious retarding 
effect, abrupt changes in direction and sharp curves should be 
avoided so far as possible. For the same diameter of pipe the 
resistance caused by a bend increases as the radius of the curve 
diminishes, but the exact relation is not accurately known. In 
the absence of sufficient experimental data the following table is 
given, as published in the catalogue of the Norwalk Iron Works 
Co.: 



Table XVIII 



Radius of elbow in terms of , 
diameter of pipe i 5 


3 


2 


1* iJ 


1 


3 

4 


1 
1 


Equivalent length of straight 
pipe in terms of its diameter. . 


7-85 


8.24 


9-°3 


10.36 12. 72 


J7-5 1 


35-09 


121. 2 



It would appear that these allowances are none too large, 
since for steam piping the frictional resistance of each ordinary 
sharp right-angled elbow is considered equivalent to that due to 
a length of straight pipe equal to forty times its diameter. How- 
ever, in putting in wrought-iron air piping of the sizes customarily 
used the bends are not necessarily so sharp as a standard right- 
angled elbow. When many sharp bends are permitted, it is 
evident that the resistance may become very great. 

Under most conditions this difficulty may be avoided by the 
exercise of proper care in the installation of the pipe lines. The 
matter should have special consideration in the stopes of mines 
timbered with square sets. As far as possible, the piping should 
be carried diagonally through the sets, bending the pipe itself 
whenever necessary, instead of using right-angled elbows. 



CHAPTER XVII 

COMPRESSED AIR ENGINES 

Compressed air may be employed as a motive power in an 
engine in two ways, viz: at full pressure or expansively. By 
working at full pressure it is understood that the air is admitted to 
the cylinder throughout practically the entire length of stroke, i.e., 
without cut-off, and that therefore nearly a cylinderful of air at 
gauge pressure is exhausted at each stroke. In this case the work 
of the air engine is roughly similar to that done in anon-expansive- 
working steam engine. Among the machines which use air in 
this way are rock-drills and simple, direct-acting pumps, without 
rotary parts. 

By the term expansive-working it is meant that the air is 
admitted to the cylinder during only a part of the stroke, and is 
then cut off and the stroke completed by the expansive force of the 
air. For operating in this way some equalizing agent, such as the 
fly-wheel, is essential, and as a rule a higher initial pressure is 
employed than when working under full pressure throughout 
the stroke. It is necessary to distinguish between complete and 
partial or incomplete expansion. When the air is used with com- 
plete expansion the operation in the cylinder is the reverse of 
adiabatic compression in a compressor, the final pressure being 
equal to that of the atmosphere. But as no condensation is 
possible with air, it follows that the lowest terminal pressure in 
the cylinder must still be sufficiently above atmospheric pressure 
to produce a proper exhaust, and to overcome the friction of the 
engine at the end of the stroke. Hence, theoretically complete 
expansion is impracticable for simple air engines of ordinary 
design. 

Most air engines work with partial or incomplete expansion, 

207 



208 COMPRESSED AIR PLANT FOR MIXES 

the air expanding adiabatically in the latter part of the stroke. 
The point of cut-off is such that the terminal cylinder pressure 
exceeds the back-pressure by an amount sufficient to cause a free 
exhaust. In the conditions here set forth, no reference is made 
to the thermal changes incident upon adiabatic expansion in the 
air cylinder. Although in principle compressed air is used like 
steam, both being elastic fluids, there is an essential difference in 
the results obtained, due to the reduction in temperature. In ex- 
panding behind the piston, a given volume of compressed air at 
a given pressure will not produce the same amount of power as 
steam under the same conditions. If two curves be constructed, 
representing the expansion of equal volumes of air and steam, from 
the same initial pressure down to pressures below that of the 
atmosphere, it will be seen that the steam pressure at all points 
of the stroke is considerably higher than the air pressure; and 
the expansion curve of the air reaches the atmospheric line much 
sooner than the steam curve. 

Fig. 89 shows an ideal card, in which the initial pressure is 
75 lbs., and the cut-off is at -g- stroke. The adiabatic expansion 
curve of the air shows that the pressure is reduced to zero gauge 
pressure when the air has expanded to 3 j times the initial volume, 
the mean effective pressure being 18.9 lbs. At the end of the 
stroke the pressure falls to 7 lbs. below atmospheric pressure. 
The steam curve, on the other hand, does not cut the atmospheric 
line until the expansion reaches 4J times the initial volume, and 
the mean effective pressure is 25.2 lbs. The lower mean pressure 
of the air is due to the development of cold during its expansion. 
The operation is the reverse of compression, and the resulting loss 
of motive power is analogous to the loss of work in the compressor 
caused by the generation of heat. Just as the heat of com- 
pression reacts upon the air while being compressed in the cylin- 
der, and produces a higher tension than that due to the mere 
reduction in volume; so conversely, when expansion takes place, 
the air, which is usually at normal atmospheric temperature on 
entering the cylinder, rapidly gives up its sensible heat, and the 
cold reacting upon the expanding air reduces its pressure faster 



COMPRESSED AIR ENGINES 



209 



TO 



60 



50 



2 40 

Z 

O 

Q. 

UJ 

DC 30 

ID 

CO 

CO 

UJ 

DC 

Q. 

30 



10 



LENGTH OF STROKE 
2 3 4 



























































































































































































"C\ 












1 










































































































































































- 
































































































































pr\ 








































































































































I 






























































l\ 














































































































'0 






















































































































































































































































































































10 






















































































































































































































































































































30 






















































































































































































































































































































°0 




























































































































































t-<V 






























































\y<* 






















































X 


& 




V * 


N 


« 


























It) 

































































































































































































































































































































































































































































































































































































































































































































Viae 


HU 


m Lii 


ie~ 


\ 
















































1 


I 


\ 



























Fig. 89. — Expansion Curves of Steam and Air. 



14 



2IO 



COMPRESSED AIR PLANT FOR MINES 



than that which is due to the increase in volume alone. More- 
over, this behavior of compressed air is independent of the initial 
temperature, since the resulting expansion curve would be unal- 
tered. In the case of steam the initial temperature is high, and 
is reduced but little during expansion from ordinary working 
pressures down to atmospheric pressure. 

A similar comparison may be made for other initial pressures 
and ratios of cut-off. In every case the mean effective pressure 
is higher for steam than for air. It follows that, to develop the 
same amount of power in a given cylinder and with the same 
initial pressure, the cut-off must be later in the stroke with air 
than with steam. 

So low are the temperatures produced by the expansion of air, 
from ordinary working pressures of sixty or seventy pounds down 
to atmospheric pressure, that for a long time the expansive use of 
compressed air was considered impracticable. In Table XIX 
are given the theoretical final temperatures of the exhaust air, in 
working with complete expansion, and also at full pressure 
throughout the stroke, for different ratios of initial to final press- 
ure, together with the theoretical efficiencies. The initial tem- 
perature is taken as 68° F.* 

Table XIX 



Ratio of 


Working With Complete Ex- 
pansion. 


Working at Full Pressure. 


Initial to 










Final Press- 










ure. 


Final Tempera- 


Theoretical Effi- 


Final Tempera- 


Theoretical Effi- 




ture. Degrees Fah. 


ciency. 


ture. Degrees Fah. 


ciency. 


2 


— 28.2 


-855 


- 8.4 


.82 


3 


- 76. 




806 


—34-5 




72 


4 


— 106.6 




782 


—45-7 




67 , 


5 


—128.2 




768 


—54-4 




63 


6 


—144.4 




758 


-59-8 




60 


7 


-158-8 




•751 


—63-4 




57 


8 


—170.8 




.746 


—66.1 




55 


9 


— 180.6 




.742 


—68. 




53 


IO 


— 189.2 




739 


—69.7 




51 



* M. Mallard, " Etude Theoretique sur les Machines a Air Comprime," p. 27. 
Robert Zahner, "Transmission of Power by Compressed Air," p. 100. 



COMPRESSED AIR ENGINES 211 

In the table it is shown that by working at full pressure 
extremely low temperatures of exhaust are avoided; but the 
efficiency of this method of using compressed air is necessarily 
much below that obtained from expansive working. It is under- 
stood that the temperatures here given are theoretical and are 
never actually reached in practice. The cold produced is modi- 
fied by several causes : (i) Some heat is transmitted from the ex- 
ternal atmosphere through the cylinder walls; (2) the re-com- 
pression of the clearance air at each stroke produces heat in the 
cylinder, to a degree that increases with the initial pressure and 
the clearance volume; and (3) the presence of even a small quan- 
tity of moisture in the air tends in some degree to raise the 
cylinder temperature. 

A few brief notes will here be given concerning the elements 
of the operation of compressed-air engines, that may be con- 
sidered more or less applicable for ordinary service, viz : working 
at full pressure, with partial expansion, or with complete expan- 
sion. Isothermal expansion may be neglected, since it involves 
the application of a sufficient degree of external heat to the air 
while doing its work in the cylinder to produce a terminal tem- 
perature equal to the initial temperature. 

1. Working at Full Pressure. This mode of using com- 
pressed air is common for engines like pumps, operating under a 
constant resistance and not provided with fly-wheels: 
Let P' =the absolute initial pressure of the air. 

V' = the initial volume of air, at the pressure P', or K times 
the volume of one pound of air used per unit of time. 

T'=the absolute initial temperature of the compressed air. 

T =the absolute final temperature of the air at exhaust, on 
expanding to atmospheric pressure. 

P = pressure of the air at exhaust. 

W = foot-pounds of work done. 
From the theory of compressed air : 

R = J (C^— C z ,) = 772 (0.2375— 0.1689) = 52.96, where J is 
Joule's heat unit, and C* and C v are the specific heats of air at 
constant pressure and constant volume. 



212 COMPRESSED AIR PLANT FOR MIXES 

As no work is done by the expansive force of the air originally 

produced by compression, W equals the volume of air used, V, 

multiplied by the difference between P' and P, or: \Y = Y'(P' — P). 

RRT 
Substituting for V its value, — =57— , as obtained from: P' V 

= KRT', 

RRT' P 

W=-p r -(P'-P) = KRT'(i-- 

p 
Giving R its value, 52.96: W = 52.96 KT' (1 - — 

2. Working with Partial Expansion. The advantages of 
using compressed air in this way may be obtained from engines 
possessing fly-wheels, provided that the cut-off be not too early in 
the stroke to avoid excessive reduction of cylinder temperature, 
or else that the air be reheated before entering the cylinder. 

In this case the values of P', V, and T' are as above. From 
the point of cut-off the air expands adiabatically down to a ter- 
minal pressure of P" and volume Y", the final temperature in the 
cylinder falling to T". On exhausting, the pressure, volume, and 
temperature become P, V, and T. The work done is composed 
of three parts, viz : 

W'=work between the point of admission and the point of 

cut-off = P V. 
W = work performed by expansion of the volume V from the 

point of cut-off to the end of the stroke =772 KC. 
(T'-T"). 
\Y'"= negative work due to back-pressure = — P Y". 

Taking the algebraic sum of these three quantities : 

W=F V + 772 K C : iT'-T")-PY" 

t~> \ ft RR I 1 ,~i, RR I 

But, as under (1): \ = — —, — and \ = 



Substituting these values of V and Y", and for R and C t . 
their numerical values of 52.96 and 0.1689: 

W = K [ S 2.96 T'- 130.4 (T'-T")-52-96 T (p)] 



COMPRESSED AIR ENGINES 



213 



3. Working with Complete Expansion. In the theoretical 
card, Fig. 90, is shown the relations of the compression and ex- 
pansion lines, the shaded portion representing the useful work 
done by the complete expansion of cold air in a motor cylinder. 




When the expansion is adiabatic, the same relations exist between 
pressures, volumes, and temperatures as were set forth in the dis- 
cussion of adiabatic compression, viz : 

The theoretical work done by complete adiabatic expansion 
may be expressed by a formula similar to that employed for com- 
pression, but with an inversion of certain of the quantities, thus: 

W-^^D"©^" 1 ]. in which 

W = the theoretical foot-pounds of work done by the expan- 



214 COMPRESSED AIR PLANT FOR MINES 

sion to atmospheric pressure of i pound (13.1 cu. ft.) of free air. 
Substituting the values of the constants, and for working at sea- 
level : 

W = 3.463 X 144 X 14.7 X 13.1X 

For example, if P' be 40 lbs. gauge pressure: 

W = 96,020. 1 — ( — —J =30,440 ft. lbs., or 2,323 ft. lbs. 

per cu. ft. of free air. 

Actual Work Done. In the above expressions no account is 
taken of the friction of moving parts of the motor engine, nor loss 
of work caused by leakage. In determining the actual work, the 
general case will be where a cut-off is employed. The relations 
between initial and terminal pressures and temperatures, for 
different ratios of expansion in a motor-engine cylinder, are 
shown in Table XX,* the points of cut-off, in tenths of the cylin- 
der stroke, being given in the first column . 

The quantities in Table XX must be further corrected for 
piston clearance and the lost volume represented by the air ports 
and passages of the cylinder, because part of the air expands into 
these clearance spaces. Therefore, the actual effect of the cut- 
off, in any given case, is found by dividing the sum of the cut-off 
plus clearance, by the cylinder volume plus clearance. For ex- 
ample, if the stroke be 10, with a cut-off of t 4 q-, and clearance of 6 
per cent., the actual volume of the cylinder, including clearance, 
will be: (10 X. 06) + 10 = 10.6. Then the ratio of actual cut-off, 
plus clearance, is 4 + .6=4.6, and the working cut-off becomes 
4.6 -mo.6 = 0.434. In this way Table XXI has been constructed, 
for use in connection with Table XX. It shows the actual cut- 
off corresponding to the different nominal points of cut-off, for 
the percentages of piston clearance named at the top of the col- 
umns. 

* This table, as well as Table XXI, is taken in part from those used by Gardner 
D. Hiscox, in "Compressed Air, its Production, Uses and Application," 1901, p. 
202. 



COMPRESSED AIR ENGINES 



215 



Table XX 
Theoretical Ratios of Pressures and Temperatures Due 
to the Expansion of Compressed Air in a Motor 
Cylinder. 





K * 




5 ai"£ 


Is 


3 <»« 


rt t/i O r 
e CD-— r 




6 


Ratio of E 

pansion 

= i-r-Cut-c 


Ratio of Me 

to Total Ab 

lute Pressure 

Entire Stro 


Ratio of Me 
to Total Ab 
lute Pressu 
During Exp 
sion only 


Ratio of 

Initial to Fi 

Temperatu 


Ratio of 

Initial to Fi 

Absolute 

Temperatu 

Due to Exp 

sion only 


Ratio of 

Initial t to Fi 

Absolute Pr 

ure for Rat 

of Expansir 


O.IO 


10.00 


0.249 


0.166 


0.391 


°-5i3 


0.039 




r 5 


6.67 




348 




2 33 




460 




578 




069 




20 


5.00 




43 6 




2 95 




518 




627 




104 




2 5 


4.00 




515 




353 




568 




669 




142 




30 


3-33 




5*5 




408 




612 




7°5 




184 




35 


2*86 




647 




460 




652 




737 




228 




40 


2.50 




706 




5io 




688 




767 




275 




45 


2.22 




757 




55« 




722 




794 




3 2 5 




5° 


2.00 




802 




604 




754 




818 




378 




55 


1.81 




842 




649 




784 




841 




433 




60 


1.67 




877 




692 




812 




862 




487 




65 


i-54 




907 




734 




839 




882 




545 




70 


i-43 




93 2 




774 




865 




902 




605 




75 


*-33 




954 




814 




889 




920 




667 



Table XXI 

Excess of Cut-off Due to Percentage of Clearance for 

the Nominal Cut-offs in Column i. 



.- 




Percentage of Clearance. 






3 


■03 


.04 


.05 


.06 


.07 


.08 


.10 


.10 


0.126 


o-i35 


J 43 


0.151 


0.159 


167 


0.182 




15 


•i75 


.184 


191 


.198 




206 


213 


.227 




20 


•223 


.231 


238 


•245 




252 


259 


•273 




25 


.272 


•279 


286 


•293 




299 


3°5 


.318 




30 


.320 


•327 


333 


•34o 




346 


352 


•364 




35 


.368 


.376 


380 


•387 




392 


398 


.409 




40 


.417 


•423 


429 


■434 




439 


444 


•455 




45 


•465 


.471 


477 


.481 




486 


490 


.500 




50 


.514 


•519 


524 


.528 




533 


537 


•546 




55 


•5 6 4 


.568 


57i 


•576 




580 


585 


.591 




60 


.612 


.615 


619 


.623 




626 


630 


•637 




65 


.660 


.664 


667 


.670 




673 


676 


.682 




.70 


.709 


.711 


7i4 


.717 




720 


.722 


•727 


•75 


•758 


.760 


762 


.764 




766 


768 


.772 



2l6 COMPRESSED AIR PLANT FOR MINES 

The theoretical terminal cylinder pressure resulting from 
adiabatic expansion may be expressed by : 

P' . i 

— — -7 — P, in which C = ratio of expansion = — : -. — 

C 1 4o6 F point of cut-off 

(see column 2, Table XX). 

For example, for a cut-off at — stroke and 65 lbs. gauge press- 
ure, the terminal pressure (above atmospheric pressure) will be: 



65 + 14.7 „ 

-^ ^ — 14.7 = 7.2 lbs. 

r, r 1.406 ^ ' ' 



2.5 ± - 4 ° 6 

The volume corresponding to the nominal cut-off is increased 
by the clearance, and adds to the mean pressure. Thus, in the 
above example, assuming the clearance to be 6 per cent., the actual 
cut-off (Table XXI) is increased from 0.4 to 0.434, of which the 

ratio, C, is = 2.3. From Table XX, column 7, the ratio of 

•434 

initial to terminal pressure, corresponding to the actual cut-off 

of 0.434, is (by interpolation) .31 ; whence: (79.7 X 0.31) — 14.7 = 

10 lbs. terminal pressure. 

Cylinder Volume Required for a Given Power. The work per 
stroke is found by dividing the foot-pounds of work to be done 
per minute by twice the number of revolutions of the engine 
(which would be determined for any given size of engine by 
the ordinary empiric rules of practice). This is substituted, with 
the initial and final pressures, in the formula for working with 
full pressure, partial or complete expansion, as the case may be, 
which is then solved for the initial volume, V, of compressed 
air used per stroke. To the theoretical cylinder volume thus 
found, the allowance for piston clearance is added, according to 
the type of engine. The proper proportion between stroke and 
diameter of cylinder is finally determined. 

The volume of free air per minute, required for an air engine, 
per indicated horse-power and for different ratios of cut-off, are 
shown in Table XXII, by F. C. Weber.* The figures given in 

* Compressed Air, Oct., 1896, p. 117. 



COMPRESSED AIR ENGINES 



217 



this table do not include the volume corresponding to piston 
clearance which may be found as already shown. 



Table XXII 
Cubic Feet of Free Air per Minute Used in Motor Engine, 

Per I, H.-P. 



Point of 








Gauge Pressures, Pounds. 




Cut-off. 
































30 


40 


50 


60 


70 


80 


90 


100 


no 


125 


1 


23-3 


21.3 


20.2 


19.4 


18.8 


18.42 


18.10 


17.8 


17.62 


17.40 


5. 
4 


18.7 


17. 1 


16. 1 


15-47 


15.0 


14.6 


14-35 


14.15 


i3-9» 


13-78 


2 
3 


I7-S5 


16.2 


15.2 


14-5 


14.2 


13-75 


J3-47 


13.28 


13.08 


12.90 


1 

2" 


16.4 


14-5 


i3-5 


12.8 


12.3 


n-93 


11. 7 


11.48 


11.30 


11. 10 


1 
3 


i7-5 


15.2 


12.9 


11.85 


11.26 


10.8 


10.5 


10.21 


10.02 


9.78 


1 

4 


20.6 


i5-6 


i3-4 


!3-3 


11.40 


10.72 


10.31 


10. 


9-75 


9.42 



In this table the air is supposed to be used without reheating, 
and at an initial temperature of 6o° F. Reheating will reduce the 

T 

volume of air proportionally to the ratio ^r, where T 2 = 459 + 6o° 

I3 

= 519 F., or absolute temperature; and T 3 = 459° plus the tem- 
perature of the reheated air on entering the motor cylinder. 
Thus, if the air be reheated to 200 F., the above ratio becomes 



5^9 
059 



- = 0.787, by which decimal the volume of air as found in 



the table must be multiplied. 

So far as mine service is concerned, it has been customary 
to consider compressed air almost exclusively as an agent for the 
operation of rock-drills, and in view of its preponderating applica- 
tion to this use its adaptability under proper conditions to the 
driving of other machines and engines is sometimes overlooked. 
Of late years, however, with improved methods of compression 
and reheating, attention has been given to employing compressed 
air for a greater variety of service; not only underground, but 
for certain portions of the surface plant of mines as well. 
Aside from cases where the disposal of exhaust steam would be 



2l8 COMPRESSED AIR PLANT FOR MINES 

troublesome, the question is largely one of comparative loss in 
transmission and the power cost of the air. 

Although not strictly in place in this chapter, reference may 
be made to what has been called the " two-pipe system" or 
" high-range compressed-air transmission," introduced some 
years ago by Charles Cummings.* 

The machine or engine using the air makes in effect a closed 
circuit with the compressor. After the air has done its work in 
the motor cylinder, it is returned to the compressor at the pressure 
of the exhaust, through a second line of piping. The return pipe 
connects with a closed chamber at the compressor, in which the 
inlet valves are placed, thus enabling the compressor to begin its 
stroke with the cylinder filled under a considerable initial pressure. 
Then, after raising the pressure to the original point, the com- 
pressor delivers the air into the main, to be used again by the air 
engine. The actual working pressure of the air engine is, there- 
fore, the difference between the pressures in the delivery and 
return pipes. Barring leakage, the same air is thus used over and 
over, the intention being that the compressor shall put back into 
the air kept in circulation the power expended in the motor 
engine cylinder. 

Though the compressor itself is not materially different from 
the ordinary forms, the two-pipe system requires a rather com- 
plicated arrangement of piping and valves for charging the 
apparatus with air at the working pressure adopted, and for 
governing the speed and output according to the rate of con- 
sumption of air. f The advantages of the system'are : a higher 
efficiency than is obtained from moderate-size compressors of the 
usual types, and less trouble from freezing at the motor engine 
by reason of the relative dryness of the air due to its higher 
tension. The efficiency increases with the pressure employed. 
In using compressed air without reheating the two-pipe system 

* Patent No. 456,941 was issued to Mr. Cummings in 1891. 

f A detailed illustrated description is given by Frank Richards in American 
Machinist, April 28th, 1898, p. 23. See also Compressed Air Magazine, Oct., 
1907, p. 4599- 



COMPRESSED AIR ENGINES 219 

is superior in principle to the ordinary mode of operating com- 
pressed-air plant. But because of the greater first cost its ad- 
vantages disappear when reheating can be adopted, and the 
single-pipe system is then found to be preferable. 

The two-pipe system is best suited for machines working 
at full pressure throughout the stroke, such as machine drills or 
simple, direct-acting pumps. When the motor works expan- 
sively the pulsations become objectionable, as a regular flow of 
air is not maintained in the return pipe. Under these con- 
ditions the inertia and friction of high-pressure air in long pipe 
lines becomes noticeable and disadvantageous. 

As the length of air pipe required for this system is doubled, 
not only may the first cost of the pipe go far toward offsetting the 
greater efficiency but, with at least twice as many joints in the 
pipe lines, the chances of loss from leakage are increased. And 
if very high pressures be used (pressures of several hundred 
pounds have been proposed), not only must the piping itself be 
heavier and more expensive, but the proportionate power loss 
from leakage is greater. For moderate distances, however, and 
when working at full pressure under the proper conditions, the 
foregoing disadvantages may be more than counterbalanced by 
the superior efficiency of the system. Though not yet in general 
use, the two-pipe system is said to have given satisfaction at sev- 
eral mines in New Mexico, Colorado, and California,* and has 
recently (1905) been proposed for use in the Johannesburg gold dis- 
trict. Some prominence is here given to the system because of 
its novel features and the probability that it may be found use- 
ful, if its disadvantages can be overcome. Reference may be 
made to a paper by H. C. Behr, published in 1905 in the Transac- 
tions of the Mechanical Engineers' Association oj the Witwaters- 
rand, in which the Cummings system is treated at length, with a 
discussion of its advantages as applied to compressed-air-driven 
pumps. 

* A. E. Chodzko, Modern Machinery (Chicago), Jan., 1899, p. 11. 



CHAPTER XVIII 

FREEZING OF MOISTURE DEPOSITED FROM COM- 
PRESSED AIR 

Reference has been made in a former chapter to the trouble 
sometimes caused by the congelation of the moisture carried 
in compressed air when deposited in the transmission pipes or in 
the ports and exhaust passages of the machine using the air. 
The presence of moisture in compressed air must be accepted as 
an unavoidable condition. Existing in the atmosphere at all 
times in greater or less quantity, when air is compressed the 
moisture is carried with it. A part of the water is deposited in 
the air receiver, but a considerable quantity still remains and will 
be brought into evidence when the proper conditions occur. 

The capacity of air for moisture depends primarily upon its 
temperature. Under ordinary atmospheric conditions 1,000 
cubic feet of air contain about one pound of water. When its 
volume is reduced in the compressor cylinder, the increase of 
heat which takes place augments its moisture-carrying capacity. 
Any subsequent decrease in temperature reduces this capacity, 
and if the air be saturated the excess of moisture is deposited. 
Volume for volume, the capacity of air for moisture is independ- 
ent of its pressure or density. That is, at the same temperature, 
a cubic foot of air at atmospheric pressure will hold in suspension 
the same weight of water as a cubic foot at 160 pounds pressure. 
But this must not be misunderstood. If a certain volume of 
moist atmospheric air be compressed isothermally, say to yV 
of its original volume, its water capacity is also reduced to tV> and 
To" of the water originally present in the air is deposited. There- 
fore, while the capacity for carrying moisture of a given vol- 
ume of air varies with the temperature, it must change also with 
any increase or decrease of pressure which changes its volume. 

220 



FREEZING OF MOISTURE DEPOSITED FROM COMPRESSED AIR 2 21 

Causes of Freezing. Certain conditions are required to cause 
freezing of compressed air: deposited moisture must be present, 
and it must be subjected to a temperature below the freezing- 
point. So long as the temperature does not fall low enough, the 
presence of moisture can do no harm. Although one of the 
recognized functions of the air receiver is to permit the deposition 
of water before the air passes into the pipes, still, unless the re- 
ceiver be extremely large, the air leaves it warm — usually even 
quite hot — and therefore carries with it considerable moisture. 
In the case of wet compressors, unless liberal sprays are used to 
attain effective cooling, the air is apt to contain more moisture 
than that from dry compressors. A well-designed injection com- 
pressor, however, not too small for its work and therefore running 
at a moderate speed, will deliver cool air which will not give 
trouble from freezing. The air having attained nearly normal 
temperature before entering the pipe-line, its moisture-carrying 
capacity undergoes but little further reduction while passing 
through the pipe, and only a small amount of additional de- 
position takes place. With dry compression the percentage of 
humidity of the intake air, and the temperature at discharge, 
determine the quantity of water carried out of the cylinder. The 
humidity, in turn, varies with the weather. Changes in the 
weather may quickly be followed by variations in the quantity 
of moisture deposited in the receiver and pipe-line. When the 
air is finally expanded in doing its work in the air engine, intense 
cold is produced as the pressure falls, and the latent heat of 
compression is absorbed. It is here that the moisture carried 
with the air into the pipes makes its appearance as frost and 
causes trouble. Watery vapor itself, depositing a light, snow- 
like frost, does not tend to clog the air passages and ports as 
much as entrained water in a finely divided state, which will 
gradually form accumulations of solid ice and choke the exhaust 
wholly or in part. 

Prevention of Freezing. The difficulties which may arise 
from the conditions just outlined are apt to be exaggerated. That 
freezing not infrequently occurs is true, but with a properly 



222 COMPRESSED AIR PLANT FOR MINES 

designed and arranged plant it may easily be avoided. Two 
things require attention : first, the air should be caused to drop its 
moisture as completely as possible before entering the main; 
second, provision should be made for draining off what deposited 
moisture remains in the pipe-line, before the air passes to the 
machine in which it is to be used. Although this is a simple 
matter, the means for accomplishing it are often neglected. Con- 
siderable quantities of water may collect in low places in the 
pipe-line and, if not blown out at intervals, will be carried into the 
ports, cylinder, and exhaust passage of the air machine and there 
freeze. 

Granting that the air leaves the receiver near the compressor 
practically saturated and still warm, it is evident that a great 
improvement in working conditions may be realized by intro- 
ducing a second receiver as close as possible to the machines 
using the air. In mining the second receiver is, of course, placed 
underground.* Before reaching it, the temperature of the air 
will have become normal, and the entrained moisture from the 
pipe-line may readily be trapped and drawn off. It may be re- 
marked that automatic water-traps are preferable to valves or 
cocks for getting rid of the water. As a rule, when the com- 
pressed air is to be used expansively, a special aftercooler should 
be introduced, placed as close as possible to the compressor. In 
any case, the receiver should be of ample size to insure the de- 
position of the moisture. The advantages of reheating the air 
before use will be taken up later. 

Influence upon Freezing of High Pressures in Transmission. 
The statements made in the first part of this chapter suggest an 
important consideration, viz: in transmitting power by air at a 
high pressure there is less liability to trouble from freezing than 
when low pressures are employed, provided that the length of 
pipe-line is sufficient to allow the air to be completely cooled and 
drained of its water while still under high pressure. At a low 
pressure a greater volume of air is required to furnish a given 
amount of power than when at a high pressure. More moisture 

* See latter part of Chapter XL 



FREEZING OF MOISTURE DEPOSITED FROM COMPRESSED AIR 223 

must, therefore, be dealt with, and at the low pressure it cannot 
be so thoroughly separated before the air is used. Suppose the 
transmission to be at a high pressure, and through a pipe long 
enough to allow the air to reach normal temperature. If the 
deposited moisture be drained away while the air is at its maxi- 
mum pressure; then, if the air be subsequently expanded down 
to a lower pressure suitable for working (with a corresponding 
increase of volume) and allowed to regain its normal tempera- 
ture, the percentage of moisture will be reduced, so that the air 
may be relatively very dry. When finally used in the air engine 
there will not be enough moisture present to cause troublesome 
freezing. 

Deposition of Moisture by Reducing Pressure. Still another 
mode of minimizing trouble from freezing is to reduce the press- 
ure of the air before it enters the cylinder of the air engine. The 
means by which this is accomplished and the results obtained 
may be illustrated by an example. 

At the Drummond Colliery, Nova Scotia, for running an 
underground pump by compressed air two receivers are used, one 
near the pump, and another 300 ft. farther back on the pipe- 
line. The air pressure in the main from the surface is 85 lbs., 
and as the proportions of the cylinders of this particular pump 
are such that so high a pressure was unnecessary a reducing valve 
was put in the pipe just before reaching the first receiver. By 
this valve the air is wire-drawn to reduce the pressure to forty- 
five pounds, which results in a deposition of nearly one-half the 
entrained water, in addition to that already deposited in the 
pipes. It is found that more moisture collects in the first than 
in the second receiver, and by this device the serious difficulty 
previously encountered from freezing at the pump has been en- 
tirely overcome.* The temperature lost by the reduction of 
pressure to forty-five pounds is regained before the air reaches 
the pump. 

* This information has been kindly furnished by Charles Fergie, superin- 
tendent of the Drummond Colliery. See also Mr. Fergie's article on the subject, in 
Transactions Canadian Mining Inst., 1896, of which an abstract was published in 
the Colliery Guardian, October 30th, 1896, p. 821. 



224 COMPRESSED AIR PLANT FOR MIXES 

Protection of Surface Piping. What precedes refers only to 
the freezing produced by internal reduction of temperature, acting 
on the moisture carried in the air. In using compressed air, even 
for mining purposes, it often becomes necessary to carry lines of 
air pipe considerable distances on the surface. To prevent con- 
densation and freezing of the moisture in winter by external cold, 
all surface piping must be protected. If exposed to temperatures 
below the freezing-point, the inside of the pipe will become 
coated with ice and its effective cross-section reduced. A serious 
diminution of area may thus be caused at low points in the pipe- 
line, where water tends to collect ; or the pipe may even be frozen 
solid in such places by the gradual accumulation of ice. Under- 
ground the temperature is rarely, if ever, low enough to render 
any protection necessary, except in cold, down-cast shafts, or in 
tunnels in which there is a strong inward draught. 

Some time ago, at one of the Butte copper mines, a simple and 
inexpensive device was employed to prevent the freezing of mois- 
ture in a long line of surface piping. The air main of a large 
compressor plant was carried on the surface some hundreds of 
feet before reaching the shaft. During the winter months it was 
at times difficult to get sufficient air pressure in the mine because 
of the partial choking up of the pipe. As the volume of com- 
pressed air was too large to be dealt with by the ordinary receiver, 
a series of old tubular boilers were placed close to the compressor 
house. The hot air, at eighty pounds gauge pressure, in passing 
through these boilers, from one to another, was cooled down 
practically to atmospheric temperature and as a consequence a 
large part of its moisture was deposited. It was found that dis- 
carded tubular boilers, when strong enough, were well suited to 
this purpose, because of the large surface presented to the cold 
outside air; especially when they are set horizontally, so that there 
is a free circulation of air through the tubes. A blower might 
be used for the same purpose in a warm climate, or the boilers 
submerged in cold water. This effectual remedy is worthy of 
adoption where the conditions are similar. 



CHAPTER XIX 
REHEATING COMPRESSED AIR 

After the warm compressed air leaves the compressor and 
receiver on its journey through the transmission line its tem- 
perature is quickly reduced to that of the surrounding atmosphere. 
The loss thus suffered could be prevented only by using the air 
immediately and before it has time to cool. But this is never 
possible in mining practice. It would be unreasonable to pro- 
duce compressed air for use close to the compressor, because of the 
loss that inevitably ensues whenever power is converted from one 
form into another. The principal object in compressing air is 
to convert the power into a convenient form for transmission to a 
distance. The facility with which the heat of compression is 
given up, however, suggests that a gain may be effected by reheat- 
ing the compressed air when it reaches the place where it is to 
be utilized. 

The process is a simple one, and by such reheating an ad- 
ditional volume of air is obtained at a much lower power cost than 
if it were produced in the compressor itself. This may be shown 
by comparing the number of heat units required to produce a 
given volume of air at a given pressure in a compressor cylinder, 
with the heat units required to accomplish the same result by 
causing the air to expand through the direct application of heat. 
Herein is the ultimate basis of comparison for determining the 
useful effect of reheating. 

Appliances for, and Results of Reheating. A number of 
methods of reheating have been actually used or proposed, the 
most important of which are as follows : (i) The air to be heated 
is passed through a cast-iron chamber or coil of pipe, exposed to a 
fire or current of hot gases or steam; (2) heat may be added 
15 225 



220 COMPRESSED AIR PLANT FOR MIXES 

within the body of air itself, such as by the combustion of fuel, the 
injection of steam or hot water, or the placing in the air pipe of an 
electric-resistance coil. 

The method most frequently employed is the one first named; 
it is preferable from a mechanical standpoint and is the most 
efficient. Those appliances in which internal combustion is 
adopted, or in which hot water or steam is the heating agent, are 
less satisfactory in practical operation, but are useful where the 
burning of fuel is not admissible. 

The following calculation,* showing the results theoretically 
obtainable by reheating, presents the matter in concise form: 

Weight of i cu. ft. of steam, at 75 lbs. gauge = o.2o8o, lb. 

Total heat units in 1 lb. of steam, at 75 lbs., produced from 
water at 6o°F. = 1151. 

Total heat units in 1 cu. ft. of steam at 75 lbs. = 1 151 X 
0.2089 = 240.44. 

To produce by compression in a steam-actuated air com- 
pressor 1 cu. ft. of compressed air at 75 lbs. gauge and 6o° F., 
about 2 cu. ft. of steam at the same pressure are required,! 
making the thermal cost of 1 cu. ft. of compressed air, at the 
above temperature and pressure, 240.44X2=480.88 heat units. 
The air is here supposed to have lost its heat of compression by 
being stored or transmitted to a distance, so that the 480.44 heat 
units represent its cost at the motor where it is to be used. 

The result of reheating may now be stated : 

Weight of 1 cu. ft. of compressed air at 75 lbs. and 6o° F. 
= 0.456 lb. 

Units of heat required to double the volume of 1 lb. of free 
air at 6o° F. = 123.84. 

Units of heat required to double the volume of 1 cu. ft. of 
compressed air at the same temperature and pressure = 123.84 X 

°.45 6 = 5 6 -47- 

Comparing the thermal cost of 1 cu. ft. of air compressed 

in a cylinder with that of 1 cu. ft. obtained by reheating: 

* Frank Richards, " Compressed Air," p. 158. 

f That is, the efficiency of the compressor is assumed to be fifty per cent. 



REHEATING COMPRESSED AIR 227 

480.88 : 56.47 : : 1 : 0.1174 
that is, the cost in heat units of the volume of air produced by 
reheating is less than J of that required to produce the same 
volume by compression. 

It is not to be expected that the theoretical result here set 
forth can be attained in practice. To effect such a saving a very 
perfect form of reh eater would have to be employed, and, the air 
after reheating pass directly into the cylinder of the engine. A 
farther conveyance of the air in pipes for even a very short dis- 
tance rapidly lowers its temperature and therefore its pressure. 

Temperatures Employed in Reheating. At a constant press- 
ure the volume of air is proportional to its absolute temperature, 
or 459 F. plus the sensible temperature above the zero point, as 
read on the thermometer. The absolute temperature of air at 
70 F. is 459 + 70 = 529°. In doubling the volume by the appli- 
cation of heat the absolute temperature becomes 1058 , and 1058 
— 459 = 599°, which is the corresponding sensible or thermo- 
metric temperature. But this temperature is greatly reduced 
by the time the air reaches the motor cylinder, and still more 
heat is lost in the cylinder before its work is done. To reheat the 
air to a temperature which would really double its volume in the 
motor cylinder itself would involve a temperature in the reheater 
much higher than 599°. But such high temperatures cannot be 
employed, because they would render impossible the proper 
lubrication of the cylinder. If the temperature be raised by the 
reheater to 400° F. a loss of, say, ioo° should be allowed for cooling 
before the air is actually used. The absolute cylinder tempera- 
ture is then 300 + 459 = 7 59°, and the corresponding added volume 
of compressed air practically available is found by the proportion : 

529 : 759 ::i :i«43 + 
That is, there has been an effective increase of about 43 
per cent, in the volume of compressed air by heating in the 
reheater to 400°. It is improbable that a higher temperature 
would be desirable in the motor cylinder, or that any material 
further increase in economy could be realized in the operation of a 
compressed-air motor. In actual practice the gain derived from 



228 COMPRESSED AIR PLANT FOR MINES 

reheating is usually considerably less than is here shown. For 
air engines taking air at nearly full stroke, such as machine- 
drills and small, single-cylinder pumps, the increase of work 
ranges from, say, thirty to thirty-five per cent., without deducting 
the cost of the fuel used in the reheater. A higher efficiency is 
shown for expansive-working engines. 

For some purposes the determination as to the quantity of 
heat to be imparted in reheating is based on the temperature at 
which the air leaves the compressor cylinder, the idea being to 
recover the heat subsequently lost in cooling. Suppose, for ex- 
ample, that the compression is practically adiabatic, as is usually 
the case in single-stage dry compressors. Taking as the unit i 
lb. of air, or 13.2 cu. ft., at a temperature of 65 F., and com- 
pressing to 70 lbs. gauge, the heat of compression * is: 

{¥' \ ° 29 ( 70+ 14.7 \ a29 

T'=T(p) .6- + 4 59°( L ^r) 

= 869 absolute temperature, and the final thermometric tem- 
perature is, 869 — 459 = 410° F. The rise in temperature due 
to compression is therefore : 

4io -6 5 = 345 F. 

If the compressed air be subsequently cooled to 65 °, its volume 

1 4-7X13 -2 

becomes: = 2.20 cu. ft. 

84.7 

In using this air without reheating and non-expansively, in a 

machine such as a rock-drill, having, say, 10 per cent, clearance, 

the work done is 

W = (2.29X144X84.7X0.9) — (2.29X144 X 14.7) = 20290 ft. lbs. 

But if the air be reheated to the final temperature of compression 

(345 ° F.), the work is : 

W' = -x 20290 = 33478 ft. lbs., and the work gained by 

5 2 4 

reheating is therefore : 

33478 — 20290 = 13188 ft. lbs, or 39 per cent. 
The thermal cost of reheating this air will be: 345°X 0.2375 

* See Chapter X 



REHEATING COMPRESSED AIR 229 

(specific heat of air at constant pressure) =81.9 thermal units 

(B. T. U.), which are equivalent to 81.9X772=63226 ft. lbs. of 

work. 

Hence the efficiency of reheating in this case is : 

13 188 

= 20.8 per cent. 

63226 

In a series of experiments carried out in connection with the 
large plant of the Paris Compressed-Air Company, and using an 
improved form of reheater, the expenditure of coke in the heater, 
for one added horse-power per hour, was only 0.2 pound, which is 
say about one-eighth of the fuel consumption of large compressors 
of the best make, with compound steam cylinders. But with 
this particular plant the above very low fuel consumption in the 
heater was probably greatly exceeded. 

A working test, conducted by Prof. Alex. B. W. Kennedy, 
on a reheater supplying air for a small motor, gave the following 
results: The air was reheated to 315 F., with a consumption of 
about 0.39 lb. coke per indicated horse-power per hour, pro- 
ducing an increase of about 42 per cent, in the volume of the 
air, and, if the indicated efficiency had remained the same as 
during the trials with cold air, there should have been a decrease 

of air consumption in the ratio =0.70. The volume of cold 

1.42 

air used (admission temperature, 83 F.) was 890 cu. ft. per 
horse-power per hour; the volume when reheated was 665 cu. 
ft., or 75 per cent.; so that the full economy resulting from 
reheating was nearly realized. In this connection Professor 
Kennedy says : " I do not doubt that the stoking of the heater dur- 
ing my experiment was much more careful than it would be in 
ordinary practice, although, on the other hand, it would not be 
difficult to design a more economical stove. If, however, the coal 
consumption were even doubled, it would only amount to 72 
lbs. per day of 9 hours for 10 indicated horse-power, the value 
of which might be 6d. or 7d. The air saved per day under the 
same circumstances would be over 20,000 cu. ft., the cost of 
which, at the high rate charged in Paris, would be 7s. 3d.' 



23° 



COMPRESSED AIR PLANT FOR MINES 



A summary of the mean results obtained from two experiments 
on the above plant with cold, and two with reheated, air show:* 

i. With cold air. Incomplete expansion, wire-drawing, and 
other such causes, reduced the actual indicated horse-power of the 
motor from 0.50 to 0.39. 

2. By heating the air to about 320 F. the actual indicated 

horse-power at the motor was increased to 0.54. The ratio of 

0.54 

gain due to reheating was therefore =1.38. 

0.39 

3. Deducting the value of about 0.39 lb. coke per indicated 
horse-power per hour, used in heating the air, the real indicated 
efficiency of the whole process becomes 0.47, instead of 0.54, and 



the ratio of gain is reduced to 



Q-47 
°-39 



= 1.205. 



These carefully conducted experiments, though not made 
with a well-designed reheater, are valuable in proving that a sub- 
stantial net gain is obtained from reheating. Where reheating is 
employed in mine practice, however, the quantity of heat im- 
parted to the air is usually much less than that indicated above. 
Good results may be obtained by the application of even less than 
ioo°F. 

The results of some experiments by Riedler and Gutermuth, 
on the consumption of reheated air, by an ordinary single-cylin- 
der eighty -horse-power engine, are given in Table XXIII. | This 

Table XXIII 



Test. 


Temperature of Air. 


Consumption 

Free Air per 

H.-P. Hr. in 

Cubic Feet. 


Indicated 
Horse-Power. 


Efficiency. 


Admission. 


Discharge. 


1 
2 

3 
4 


264. 2° F. 
305-6 
320.0 
338-o 


69. 8° F. 

84.4 

95-0 

120.2 


462.77 

43 1 - 00 
418.55 
432.12 


72.3 
7 2 -3 
72-3 
65.0 


0.89 
.90 
.91 
-87 



* " Experiments upon the Transmission of Power by Compressed Air in Paris." 
Van Nostrand's Science Series, No. 106, p. 35. 
f Wm. Cawthorne Unwin, ibid., p. 104. 



REHEATING COMPRESSED AIR 



231 



engine, with Corliss valve gear, was originally designed and used 
as a steam engine, and no changes were made for adapting it to 
work with compressed air, except that the cylinder was jacketed 
by the hot air on its way to the valve chest. The initial pressure 
was 95.5 lbs. absolute and the temperature of the air in the re- 
heater did not exceed 338 F., at a coke consumption of 0.176 
lb. per horse-power hour. 

Construction and Operation of Reheaters. The reheater em- 
ployed in the experiments referred to in the preceding section was 
that in use some years ago in connection with the Paris plant. 
It consisted of a double cylindrical box of cast-iron twenty-one 
inches diameter by thirty-three inches high, over all, enclosed in a 




Fig. 91. — Leyner Compressed Air Reheater. 

sheet -iron casing. The air under pressure traversed the annular 
space between the inner and outer cylinders, being compelled by 
baffle-plates to circulate in such a manner as to come into con- 
tact with the whole heating surface. The products of combustion, 
from a coke fire in the inner cylinder, passed downward over the 




232 COMPRESSED AIR PLANT FOR MINES 

exterior surface of the annular air chamber on their way to the 
chimney. A heater of this size served for a ten to twelve horse- 
power motor. 

In another form of reheater the air is passed through a coil 
of wrought-iron pipe, enclosed in a cylindrical casing. A large 
heating surface is thus obtained, but wrought-iron pipe is ob- 
jectionable because it burns out 
rapidly unless the fire is kept mod- 
erate. The conditions are materi- 
ally different from those to which 
boiler tubes are subjected, since the 
air tubing is denied the cooling 
effect of the water. Cast-iron coils, 
on the contrary, such as those of the 
Fig. 92. — Cast-iron Coils, Ley- Leyner reheater (Figs. 91 and 92), 

ner Reheater. , n rpir tt i_ j 

stand well. I he U-shaped pipes 
are made in separate sections, bolted together as shown, with as- 
bestos packing in the joints. By varying the number of units 
any desired capacity can be obtained, and a broken or burned- 
out section is readily replaced. 

The Sergeant reheater (Fig. 93) consists of two concentric 
cast-iron shells, bolted together, one within the other, the joints 
being packed with asbestos gaskets. The inner chamber forms 
the top of the fire-box. In shape this reheater is a truncated 
cone, set on a cylindrical fire-box, the cold -air main being con- 
nected by a flange coupling at the top and the hot air discharged 
near the base. This heater measures 42 ins. outside diameter 
at the base by 54 ins. high, with a grate 19 ins. diameter. It 
is stated that 340 cu. ft. of free air per minute, at 40 lbs. pressure, 
can be heated to 360 F., with a gain of 30 to 35 per cent, in the 
energy developed. If more than 400 cu. ft. of free air per minute 
are to be reheated, 2 or more heaters of this size should be set in 
series, the air passing from one to another, allowing a maximum 
of 400 cu. ft. for each. 

Reheaters of the cast-iron-shell type, in which the inner and 
outer shells are subjected to considerable differences of temper- 



REHEATING COMPRESSED AIR 



233 



ature, and except when of small size the upper and lower ring 
joints between the shells are difficult to keep tight.* In the 
Rand reheater (Fig. 94) the castings are more complicated in 
shape, the air passing between them in a thin sheet, from the in- 
let on the side to the discharge at the top of the central dome. To 
provide for expansion and contraction, the lower joint above the 




Fig. 93. — Sergeant Reheater. 

fire-box is provided with a stuffing ring and packing, shown in the 
cut. There is still a tendency to leakage, however, if the fire be 
very hot. 

The Sullivan reheater (Fig. 95) is quite different in design 
from those described above, consisting essentially of a vertical 
coil of cast-iron piping, or hollow rings, encased in double sheet- 
steel shells, the space between the latter being filled with asbestos. 

* Sibley Journal of Engineering, 1904. 



234 



COMPRESSED AIR PLANT FOR MINES 



Below is the grate and combustion chamber, the gases from which 
pass through the spaces between the air rings. To minimize 
leakage, the centers of the rings are joined by malleable-iron 
nipples, so that all expand and contract together. These heaters, 




Fig. 94. — Rand Reheater. 



usually designed for burning coal, coke, or wood, are made in 
3 sizes, for 345 to 800 cu. ft. of free air per minute, having from 3 
to 7 rings, and measuring from 5 ft. 8 ins. to 7 ft. 6 ins. in height, 
by 33 ins. outside diameter. 

Internally fired reheaters — those in which the air is heated by 
direct contact with the fire — have hitherto been unsuccessful, 



REHEATING COMPRESSED AIR 



2 35 



because dust and injurious products of combustion are carried 
by the air into the cylinder of the air motor. This trouble, of 
course, does not exist to the same extent when gasoline or other 
oils are used, instead of solid fuels, nor in the electric reheater, 
which, however, has thus far had 
but a limited application. 

A fault of most reheaters as 
built at present is that there is no 
provision for regulating the heat ac- 
cording to the variation in con- 
sumption of air, such as is un- 
avoidable in applying reheating for 
machine drills, channellers in 
quarry work, hoisting engines, and 
other intermittently operating ma- 
chinery. This want of regulation 
evidently is not so important for 
constant-running engines, such as 
pumps. 

As the air chamber, of what- 
ever shape, in all of the externally 
heated or "dry" reheaters, forms in 
reality a part of the air main, 
reheating can increase the press- 
ure only in a small degree. Its 
real effect is to increase the volume 
of air, which tends to back up 
in the main, reducing incidentally 
the velocity of flow and there- 
fore the loss of pressure due to friction. The reheater should 
always be placed as close as possible to the machine using the air. 
This is readily done with stationary engines, like pumps or 
hoisting engines; and even in the case of movable machines, like 
quarry channellers, the reheater may be set on the same carriage 
or bed-frame. If it be necessary, however, to convey the heated 
air some distance, the temperature may be quite effectually 




Fig. 95. — Sullivan Reheater. 



236 COMPRESSED AIR PLAXT FOR MIXES 

maintained by covering the pipe with non-conducting material, 
as is done with steam piping. 

Sometimes when the air-engine cylinders are compounded, 
the exhaust from the high-pressure cylinder is passed through 
a second reheater before going to the low-pressure cylinder. A 
further benefit may be derived by injecting into the reheater a 
very small quantity of water. The specific heat of water is high 
as compared with the specific heat of air; also such part as is 
converted into steam gives up its latent heat in the motor-engine 
cylinder and prevents trouble from freezing, even when a high 
rate of expansion is employed. For the same reason, benefit 
may be derived from injecting a little warm, or even cold, water 
into the compressed-air feed-pipe of an air motor. Water used 
in this way acts incidentally as a mechanical scourer, in washing 
away accumulations of ice tending to form in the ports. 

It will be seen from the construction of reheaters that the 
calorific power of the fuel burned in them is not economically 
utilized. The flue loss i,s large for the same reasons that apply 
to the work of ordinary shell or tubular boilers. But the thermo- 
dynamic advantage gained is so considerable that the low 
efficiency of the reheater itself, in burning the small quantity 
of fuel required, becomes of secondary importance. 

Use of Reheaters for Underground Work. In the ordinary 
operations of mining the reheating of compressed air can have 
only a limited application. By far the most important use of 
compressed air in mining is for operating machine drills. Up to 
the present time there are relatively few mines where it is em- 
ployed for any other purpose. But it is evident that for portable 
machines like rock-drills, continually being shifted from place to 
place underground, the use of reheaters in most cases is economic- 
ally out of the question. Not only would a number of them be 
necessary, but they would have to be moved about and kept close 
to the drills, to prevent the reheated air from losing its heat and 
temporary increase of volume. 

For stationary engines, however, such as underground pumps, 
hoists, rope-haulage engines, etc., and wherever the reheater can 



REHEATING COMPRESSED AIR 237 

be placed permanently close to the air engine, reheating in mines 
may be successfully applied. The idea that it is useful mainly 
in preventing the accumulation of ice in the exhaust ports and 
passages of the air engine is not an uncommon one; but as a 
matter of fact the prevention of freezing is merely incidental to a 
decided gain in efficiency. In underground work it may be 
difficult to arrange for burning fuel under a reheater, notwith- 
standing the small quantity required, because of the resulting 
vitiation of the mine atmosphere. Also, in gassy collieries re- 
heaters cannot well be used, though sometimes the products of 
combustion may be turned into an upcast airway, or even allowed 
to escape into the mine workings, when the heater is small and the 
active circulation of large volumes of air is maintained. Where 
the conditions underground are such that strong combustion is 
not allowable and only a small quantity of fuel can be burned in 
the reheater, it will still be found that some advantage is obtain- 
able from air engines by a very slight added temperature — say, 
only 2 5 to 50 F. In this connection it may be noted that the 
use of the internal electric reheater, already referred to, in which a 
resistance coil is placed in an enlarged section of the air main, 
does away with the difficulty of disposing of the products of com- 
bustion of fuel and would be especially useful in gassy collieries. 
Another mode of applying electric reheating is to wrap the resist- 
ance coils around a short length of the air pipe. 

At the North Star Mine, Grass Valley, Cal., the plan has been 
adopted of placing a reheater on the surface near the shaft mouth 
and carrying the compressed air underground by a pipe covered 
with non-conducting material. Fairly satisfactory results are thus 
obtainable, with the advantage of avoiding the burning of fuel 
in the mine. But while some saving can be realized in this way 
for moderate distances — say of a few hundred feet — it would be 
economically out of the question for long transmission lines. 
This arrangement suggests the caution that non-conducting 
covering should always be used for the pipe from reheater to air 
engine, however short the distance. In a case on record,* where 

* Richards, American Machinist, Feb. 28th, 1895. 



238 COMPRESSED AIR PLANT FOR MINES 

this distance was only 20 ft., but no pipe covering provided, the 
gain in power realized was only 12 per cent., though the absolute 
temperature of the air was increased at the reheater 38 per cent., 
with of course the same theoretical increase of volume. The 
air used for operating an underground pump at another Cali- 
fornia mine is reheated by steam conveyed from the surface. 
Steam may thus be used to greater advantage than if employed 
directly in the cylinder of a pump; for, in condensing, the la- 
tent heat otherwise lost is utilized in raising the temperature of 
the air and is so converted into work. All devices of this kind, 
however, must be classed as makeshifts. 

In recent years several mine plants have been erected at which 
compressed air has been used even for operating surface hoisting 
engines — for example, at the Lightner Mine, Calaveras Co., 
Cal. One of the units of a battery of boilers is adapted as a 
reheater. The compressed air passes from the receiver into a 
section of perforated pipe submerged just below the surface of the 
hot water in the boiler, and is thence led to the hoisting engine. 
By means of a large receiver capacity, quite satisfactory results 
are secured, notwithstanding the intermittent work of the engine. 

In connection with the method of reheating referred to above 
the results may be given of a number of experiments made by 
Prof. J. T. Nicholson, in reheating air from the Taylor Hy- 
draulic Air Compressor, at Magog, Ontario. The air was used in 
a 27-horse-power Corliss engine, at a pressure of 53 lbs. There 
were 5 tests, as follows: 1. With cold air. 2. Reheating by 
means of steam injected into the air main before reaching the 
engine. 3. The compressed air was passed into a steam boiler, 
and heated by mixing with the water and steam. 4. The com- 
pressed air was blown upon the surface of the water in the boiler, 
and heated by mixing with the steam. 5. The air was passed 
through a tubular reheater, fired by coke. 

Without reheating, 850 cu. ft. of free air were used per in- 
dicated horse-power hour. By reheating in the boiler, a mix- 
ture of 10 to 15 lbs. of steam with the air reduced the consump- 
tion of air from 850 cu. ft. to 300 to 500 cu. ft., per indicated 



REHEATING COMPRESSED AIR 239 

horse-power hour. Thus, 1 added horse-power was obtained 
by wet heating, at an expenditure of from 1 to 1.3 lbs. of coal 
per horse-power hour. 

By dry heating in the coke-fired reheater, the air was raised 
to 287 F. At this temperature, 640 cu. ft. of free air were re- 
quired per horse-power hour, or 2iocu. ft. less than with cold air, 
the saving in the quantity of air being about 25 per cent. By 
burning in the reheater 47 lbs. coke per hour, ico horse- 
power in cold compressed air was raised to 133 horse-power, 
making an expenditure of 1.42 lbs. coke per hour for each added 
horse-power. These results indicate that the reheater used was 
not very efficient. But though the fuel consumption was much 
greater than in Professor Kennedy's test, previously described, 
it is still far lower than is attainable in the most efficient engine 
and boiler practice. 

In a paper by Clarence R. Weymouth, on "Reheating Com- 
pressed Air with Steam,"* a detailed discussion is given of the 
thermodynamics of this mode of procedure, with deductions as 
to its efficiency. The author considers the cases of injecting 
steam into the air main, and of passing the compressed air through 
a steam boiler, giving the results in tabulated form. 

* " Compressed Air Information." Edited by W. L. Saunders. 



CHAPTER XX 

COMPRESSED AIR ROCK-DRILLS 

It is not intended here to discuss in detail the different makes 
of compressed-air drills, nor the variations in their construction 
and operation. The aim is rather to consider the principles on 
which "they work, together with questions as to their consumption 
and mode of using the compressed air. 

Though it is a well-known fact that compressed-air drills are 
uneconomical machines in consumption of power, it is difficult to 
reach definite conclusions as to their efficiency. The actual useful 
work — employing this term in its ordinary mechanical sense — done 
by a machine drill in making a hole of given depth and diameter in 
a rock of given hardness, toughness, and general physical character 
cannot be determined absolutely. All that is really known is that 
the drill requires a certain volume of air per minute, which has 
been furnished by the expenditure of a certain average indicated 
horse-power at the compressor. Comparative figures of work 
done, in terms of speed of drilling in a given rock and per cubic 
foot of free air consumed, are often published and are useful as 
far as they go. In fact, this is the only practical basis for estimat- 
ing their efficiency. But, even in this sense, the propriety of 
accepting the results obtained, as accurately representing the 
value and efficiency of machine drills as compared with various 
forms of air or steam engines, may well be questioned. 

In their operation rock-drills differ greatly from other com- 
pressed-air machines, because the personal element of the skill 
and experience of the drill-runner exerts so important an influence 
upon the amount of work accomplished, and because the rate of 
drilling is so greatly modified by the physical and mineralogical 
character of the rock, together with the purely adventitious occur- 

240 



COMPRESSED AIR ROCK-DRILLS 241 

rence of cracks, slips, and fissures. A skilful drill-runner will in- 
evitably do more work per shift, under the same conditions, than 
an inexperienced man, and he will make a faster rate of advance 
in a rock with which he is specially familiar than if called on 
to operate a machine in rock that is new to him. 

Therefore, though mechanical efficiency, pure and simple, is 
the basis upon which machines in general are compared, in the 
case of compressed-air drills it is not the only consideration, 
nor is it the most important. Their efficiency of operation is 
subordinate to the attributes of strength, simplicity of construc- 
tion, portability, durability, ease, and readiness with which re- 
pairs may be made and capacity for work in terms of depth of 
hole drilled per unit of time. They must be capable of with- 
standing hard and often unintelligent usage. The strong point 
of compressed-air drills is their ready applicability in the special 
and peculiar field of work for which they are designed. In 
possessing a cylinder, piston, and valve, the drill roughly re- 
sembles a steam engine, but there the likeness ceases. Severe 
shock and vibration are essential accompaniments of its work. 
No fly-wheel is admissible, or other means of storing up and 
equalizing the power, and the service demanded from the rock- 
drill is therefore totally different from that performed by ordinary 
engines. 

The low theoretical efficiency of the compressed-air drill is 
due mainly to the fact that air is admitted to the cylinder prac- 
tically throughout the full stroke. As a consequence, the valve 
motion bears a strong resemblance to that of many of the single- 
cylinder, direct-acting pumps. Expansive use of the air to any 
extent is neither advisable nor practicable, both because of the 
undesirability of introducing complexity of mechanism in ma- 
chines subjected necessarily to rough usage and because of the 
difficulty of adapting cut-off gear to the variable length of stroke 
required. Owing to the nature of its work, the drill cannot be 
kept always at full stroke. While in operation it is often neces- 
sary to feed the machine so far forward that the actual length of 
stroke may be little more than one inch, and the valve motion 
16 



242 COMPRESSED AIR PLANT FOR MIXES 

must still be capable of reversing promptly. A sharp, quick 
reversal of the stroke is essential. The useful work is done on 
the forward stroke, in striking the blow. If the valve be thrown 
too soon, the stroke of the piston will be shortened; if too late, 
the piston may strike the cylinder head. For these reasons, it is 
impracticable with machine drills to attain the economy resulting 
in other air motors from using the air expansively. Incidentally, 
the use of air at full stroke is of some advantage, because, in ex- 
hausting at high pressure, the exhaust air issues from the port at 
a high velocity, and its force, combined with the development of 
some heat from friction, in a measure prevents troublesome ac- 
cumulation of ice, in case the air is moist. The freezing, if any, 
is at least confined to the exterior portion of the exhaust port, 
whence it is easily removed. 

Consumption of Air. By reason of the irregularity of the 
work of machine drilling, and the fact that in mining or other 
rock-excavation work a number of drills are always operated by 
the same compressor plant, few figures are available as to the 
actual air consumption of a single machine. Average figures, 
however, are the only really useful ones. It is customary to base 
the duty on the consumption of free air per minute, the quantity 
necessarily depending on the size of the machine, air pressure 
supplied by the compressor, character of the rock, and the pro- 
portion of the total time actually occupied in drilling. It is 
evident that the compressor capacity for a single machine is 
greater than the average required for a number of machines. 
With a large number, the delays to which each is subject, for 
setting up or shifting, changing bits, stoppages caused by the bit 
sticking in the hole, etc., make it improbable that all of them 
will be in simultaneous operation, save in rare instances; hence, 
the average allowance of air for each may be reduced. Mo- 
menta]*}' or occasional peaks in the load on the compressor, when 
an unusual number of drills happen to be working simultaneously, 
may be disregarded; or at least need not be provided for by in- 
creasing the compressor capacity. 

Rock-drills of different makers, even when of the same diam- 



COMPRESSED AIR ROCK-DRILLS 



243 



eter of cylinder, vary in their consumption of air, and reliable 
figures are not easily obtained. Table XXIV, showing the volume 
of free air per minute required for one drill, is based on a com- 
parison of the statements of several manufacturers, checked by a 
few recorded tests. It may be taken to represent, within reason- 
able limits of error, the results of actual practice for machines in 
good order. No allowance is made for the preventable loss of air 
in leaky pipes, nor for frictional loss of pressure in transmission 
(see Chapter XVI). 

Table XXIV 

Cubic Feet of Free Air per Minute Consumed by One 

Drill at Sea-Level 



Gauge 






Diameters of 


Drill 


Cylinder in Inches. 






Press- 


















ure. 




























2 


2& 


2V2 


2% 


3 


fA 


3& 


3# 


3 1 /* 


3 5 A 


4% 


5 


60 


58 


63 


70 


82 


90 


97 


100 


105 


114 


118 


135 


155 


70 


62 


72 


80 


92 


104 


112 


115 


118 


130 


135 


152 


174 


80 


70 


80 


88 


103 


ii5 


125 


130 


135 


142 


153 


173 


205 


90 


7« 


«7 


95 


115 


128 


137 


141 


148 


16.S 


173 


194 


222 


100 


«5 


96 


108 


126 


140 


151 


155 


161 


176 


184 


2IO 


250 



When a number of drills are operated by the same plant, the 
compressor capacity for furnishing the total average quantity of 
free air required per minute, at sea-level, may be found approxi- 
mately by the following table of multipliers: 

Table XXV* 



Number of drills 


1 


2 


3 


4 


5 


6 


7 


8 


9 


10 


Multiplier 


1 


1.8 


2-7 


3-4 


4.1 


4-8 


5-45 


6.1 


6.7 


7-3 


Number of drills 


11 


12 


i5 


20 


25 


3° 


35 


40 


5o 


60 


Multiplier 


7-8 


8-4 


10.3 


12.8 


15-1 


i7-3 x 9-7 


22.0 


26.5 


30-5 



The required capacity of the compressor is found by multiply - 

* Based on comparison of several tables given by manufacturers. 



244 COMPRESSED AIR PLANT FOR MIXES 

ing the cubic feet of free air per minute consumed by a single drill 
(as given in Table XXIV), by the multiplier corresponding to the 
number of drills operated (Table XXV). 

It will be understood from what precedes that the figures 
in the tables cannot be taken as exactly applicable to all cases. 
Several other modifying factors may here be summarized : 

(i) The Kind of Work. The time required to set up the drill 
depends greatly on the shape of the working, whether a tunnel or 
drift, a shaft, stope, or open cut. If the floor and roof, or the side 
walls, of a mine opening are irregular or loose, much time may be 
lost in shifting the machine and setting it up, according as it is 
mounted on column or tripod. 

(2) Character of the Rock. This also influences the consump- 
tion of air. In hard rock the rate of advance in drilling is slower 
than in soft, so that the machine makes longer continuous runs. 
Less total time is occupied in shifting and setting up for drilling 
the successive holes of a round, and the consumption of air per unit 
of time is therefore greater. Though this increase is partly offset 
by the fact that the bits are more quickly dulled in hard rock and 
must be changed at shorter intervals ; still, in very hard ground 
the machines may be kept running with but few and short in- 
termissions. In soft rock, on the other hand, though the actual 
speed of drilling is greater, there are apt to be more frequent 
delays due to rifling of the hole and sticking or " fitchuring " of 
the bit. On the whole, for hard rock it is advisable to provide a 
greater compressor capacity than is given in the tables. The 
compressor will then be able to run at a slower speed, thus avoid- 
ing excessive heating in cylinder and receiver. In general, the 
time actually occupied in drilling will vary for each machine 
from, say, 4 to 6 hours out of an 8-hour shift. 

(3) Physical Condition of the Drill. The importance of this 
matter may be overlooked. The figures given are for new 
machines, or those in thoroughly good order. More air is con- 
sumed by old drills, whose valves and pistons are so worn that 
they do not fit closely. Even in the case of drills in fair average 
condition, this is clearly shown by the fact that the exhaust, in- 



COMPRESSED AIR ROCK-DRILLS 245 

stead of being short and sharp, is nearly continuous. A large 
allowance must be made for old machines. 

If definite values could be assigned to these different items, 
estimates of air consumed per drill could be made in conformity 
with any given set of conditions. To do this is manifestly im- 
possible, but a few general data relative to averages for an entire 
shift's work have been put on record by Messrs. J. E. Bell and 
L. L. Summers, as the result of a series of experiments (Mining 
and Metallurgy, Feb. 1st, 1901). For a 3-in. drill, the volume of 
free air required per shift of 8 hours is as follows, the gauge 
pressure being ioo lbs.: 







Table XXVI 






Elevation. 




Cubic Feet of Free Air. 




Per Shift of 8 Hours. 


Per Minute. 


Sea-level 


25,000 to 42,000 
30,000 " 49,000 
35,000 " 60,000 


52.1 to 87.5 
62.5 " 102.0 
73.0 " 125.0 


5,000 ft 

10,000 ft 



These figures include all deductions, for whatever cause, cov- 
ering delays and stoppages as well as the actual drilling time. 

Taking the various allowances into account, and applying 
them to Tables XXIV and XXV, the following results, obtained 
in an elaborate test made at the Rose Deep Mine, Johannesburg, 
South Africa,* will be found in fairly close agreement with 
what precedes. The average number of drills (Ingersoll-Ser- 
geant), of several different sizes, kept in operation during the 6- 
hour test, was calculated to be equivalent to 30.9 drills, 3J in. 
diameter of cylinder. The average duty per drill was 4 ft. 
5-}-^ ins. of hole per hour (diameter of hole not stated). 
Average air pressure, 69.83 lbs. Free air used per drill per min- 
ute, 81.08 cu. ft. It is fair to assume that most of these drills were 
more or less worn, or at least not in perfect condition. Accord- 
ing to the tables, the average free-air consumption for 30.9 drills 

* L. I. Seymour, South African Association of Engineers, 1898. 



246 COMPRESSED AIR PLANT FOR MIXES 

should have been about 60 cu. ft. per minute, or about 15 per cent, 
less than that shown by the test. This difference is accounted for 
in part by the altitude above sea-level. It may be added that the 
horse-power per drill developed in the steam cylinders of the com- 
pressor was 12.72. But as the work done during the 6-hour 
test was approximately equal to that usually accomplished in 
8 hours of regular work, the actual horse-power per drill under 
normal conditions in this mine may be taken as 12.72 X-§- = 9.54. 
The air piping in this case was known to be remarkably free 
from leaks. 

Another test run, on 75 drills, 3 J in. diameter, was made 
about 3 years ago at the Champion Iron Mine, Mich.* At 
78 lbs. normal gauge pressure the average air consumption for 
the day shift, throughout a period of 1 month, was 67.1 cu. ft. of 
free air per minute. The air pressure usually dropped consider- 
ably, however, when work was in active progress. According 
to the tables, 75 drills should have used an average of about 58.5 
cu. ft. of free air per minute, or 13 per cent, less than shown by 
the test. 

Air Pressure for Machine Drills. The evidence adduced from 
recorded tests shows conclusively that a low air pressure is un- 
economical. Both the force of the blow and the number of 
strokes per minute fall off, resulting in a marked decrease in the 
footage of hole drilled. While it is probable that drilling in soft 
rock does not require so high an air pressure as for hard, it is 
found on the whole that the best results are obtained by a pressure 
of from 70 to 80 lbs. Practice of late has tended toward the 
use of higher pressures, up to 90 lbs. or even more; but, grant- 
ing that more work in some kinds of rock may be done by em- 
ploying a heavier pressure than, say, 80 lbs., the life of the drill is 
shortened and the cost of repairs increased. The customary 
nearly uncushioned blow, under a heavy air pressure on hard 
rock, becomes very destructive to the machine, and the bits them- 
selves do not stand so well. They are dulled sooner and are 
more apt to chip. 

* Engineering and Mining Journal, May 18th, 1905, p. 937. 



COMPRESSED AIR ROCK-DRILLS 



247 



The influence of air pressure, as well as the questions relating 
to air consumption per drill, are further illustrated by a number 
of tests made several years ago in the South-African gold district.* 
The rock in which the tests were made was red granite, a large 
block of which was embedded in concrete. A quarry bar was 
used for mounting the drills. All holes were drilled vertically, 
with abundance of water. Two receivers were employed, with 
a combined capacity of 757 cu. ft., the pressure for each run 
being raised by the compressor to 80 lbs., after which the 
receiver was shut off. A single machine at a time was operated, 
the run continuing until the receiver pressure dropped to 70 lbs. 
The drill was then stopped, and the depth and diameter of hole 
measured. Similar runs were successively made with pressures 
from 70 to 60, 60 to 50 lbs., etc. The capacity of the receiver, 
in terms of cubic feet of free air, was calculated for each in- 
dividual run and pressure, correction applied for temperature, 
and the air consumed based on the volume of free air at 70 F. 
and 24.8 ins. of barometer (equivalent to an altitude of 5,000 ft.). 

Eliminating several results of such runs as indicated erratic 
behavior of the drills, probably due to being in poor condition, 
a test of 13 drills, t>\ i ns - diameter with 3-in. bits gave the 
following averages : 

Table XXVII 





Air Pressure, Pounds. 




80-70 


70-60 


60-50 


50-40 


40-35 


Linear inches drilled per min 


i-3 

124. 

95-3 
T-3-3 


1.1 
117. 
106.4 
14.8 


1.0 
100. 
100. 
13.8 


0.6 
70. 
116. 4 

15.0 


o-5 
60. 


Cu. ft. free air per minute 


Cu. ft. free air per linear in. of hole . 

Ditto per cu. in. of hole 


120. 
16.6 







Each run occupied about 6 minutes. Some of the average 
results are not consistent, and the individual figures of course 
showed still greater variations. These were due to a variety of 

* J. B. Carper and others, Mechan. Engineers Assoc, of the Witwatersrand, 
1904. (Abstract in Mines and Minerals, Sept., 1904, p. 64.) 



248 COMPRESSED AIR PLANT FOR MINES 

causes, such as lack of uniformity of the rock, differences in 
temper and sharpness of bits and, in a measure, the personal 
equation of the drill-runners, each of whom " was selected by the 
agent of the maker of the drill." The rather lengthy paper from 
which these data are taken includes many tables, giving details 
of the tests of machines of different makers, and is to be recom- 
mended for the thoroughness with which the work was carried 
out. Among other points, the importance of the question of air 
pressure is clearly demonstrated. 

Valve Motion. The valve motion of most compressed-air 
drills is either of the tappet or spool type. In the tappet drills 
the throw of the valve is positive, depending directly on the 
stroke of the piston. The throw of the spool valve, on the other 
hand, is produced indirectly by the introduction of a system of 
small, auxiliary ports, connecting the ends of the valve chest 
with the cylinder, these ports being opened and closed by the 
movements of the main piston. 

In dry, dusty mines it is generally found that the tappet valve 
gives the better service. When a compressed-air drill is not in use, 
and disconnected from the air hose, dust and grit are likely to enter 
through the ports, passing thence into the valve chest and cylinder 
on resuming drilling. The wear and consequent looseness in the 
fit of the moving parts thus caused is apt to have a more un- 
favorable effect on the operation of the spool than the tappet 
valve. Leakage of air past the valve or piston prevents proper 
action of the auxiliary ports, not only producing irregularity in 
reversal and shortening of the stroke, but diminishing the drill's 
efficiency. It is true that the tappet valve involves the use of one 
extra part and, in case of the three-arm tappet, breakage is not in- 
frequent. But while the spool valve is strong and reliable, ex- 
perience indicates that in dusty mines at least the mainte- 
nance cost of the spool-valve drill is higher than that of the 
tappet drill. 

The maximum force of blow is attained by drills which take 
air throughout the full forward stroke, i.e., without cut-off, and 
the best drills are designed to work in this way. On the forward 



COMPRESSED AIR ROCK-DRILLS 249 

stroke the valve is not reversed until the blow is delivered, the 
exhaust being free, with but little back-pressure on the piston. 
Cushioning was formerly made a feature of some rock-drills, with 
the idea of reducing shock, but it is now recognized that the 
efficiency is increased by delivering an uncushioned blow. It is 
possible for a drill so designed to strike too heavy a blow in very 
hard rock, but the remedy then is to feed the drill -head down, so 
as to work with a shorter stroke. 

On the back stroke cushioning is desirable, to ease the re- 
versal and prevent injury by the piston striking the rear cylinder 
head. The back-stroke cushion is produced by cutting off the 
exhaust before the end of the stroke. Only enough power needs 
to be developed on this stroke to overcome the resistance due to 
the weight of the moving parts, and the frequent tendency for the 
bit to stick fast in the hole. 

In ordinary machine drills, the piston speed should not be too 
great — say, not much over 350 to 375 strokes per minute. The 
relative speeds of stroke do not constitute a proper basis for the 
comparison of efficiencies. To give effect to the blow, the weight 
of the moving parts must be relatively great, and a very high 
speed would be attended by excessive wear and breakage. These 
conclusions do not apply, however, to the numerous small air 
hammer drills which have come into favor in the past few years. 
They are held in the hands of the operator, weighing only from 
fifteen to thirty pounds and are useful for holes of small depth 
and diameter, as in narrow stope work, block-holing, and some 
kinds of quarrying. As the name implies, the piston or hammer 
is the only reciprocating part, the blow being delivered upon the 
inner end of the bit shank. The hammer drill strikes a light 
blow, some of them at the rate of 2,000 to 3,000 or more strokes 
per minute. Thus the weight of the moving parts is small, 
and the inertia moderate. 



CHAPTER XXI 
OPERATION OF MINE PUMPS BY COMPRESSED AIR 

It is intended here to deal only with that part of the extensive 
subject of mine drainage which has to do with the employment 
of compressed air as a motive power. Under this head there are 
three general forms of apparatus: 

i. Direct-acting pumps, single-cylinder, duplex, or compound. 

2. The air-lift pump. 

3. Pneumatic displacement pumps. 

In this chapter the first class only will be considered. 

Simple, Direct-acting Pumps. Notwithstanding the general 
similarity in the behavior of steam and compressed air, when used 
in the cylinders of direct-acting pumps there are some important 
points of difference. By first considering briefly the construction 
of the types of pump in common use the results obtainable from 
the employment of compressed air can best be set forth. 

The development of the direct-acting pump dates from Henry 
R. Worthington's invention in 1841; and the greater part of all 
the pumping in the mines of this country, and much of it in other 
countries also, is done by pumps of this class. The cylinders are 
set tandem, the power being transmitted from the steam to the 
water cylinder through a piston-rod common to both. As there 
are no rotating parts, the length of stroke is controlled by the ad- 
mission and exhaust of the steam. In all the simple pumps the 
valve motion involves the use of an auxiliary valve, whose move- 
ments are governed by the reciprocating movement of the piston, 
and which in turn operates the main valve. The duplex form 
consists essentially of two simple pumps, set side by side, with an 
inter-dependent valve motion ; that is, the valve of each is ope- 
rated positively, through a system of levers, by the movement of 
the piston of the other side. 

250 



OPERATION OF MINE PUMPS BY COMPRESSED AIR 25 1 

Though direct-acting pumps are strong and reliable, simple 
in construction, and occupy but little space, they are extremely 
uneconomical machines, unless the steam cylinders are com- 
pounded. It is hardly necessary to say that this ought not to be 
the case. Pumping is an operation that should be conducted 
economically, especially in connection with mining, where the 
pumping of water is classed as "dead work." Moreover, the 
conditions in themselves are not unfavorable. A pump works 
under a practically constant load, from the beginning to the end 
of each stroke, the only necessary variation— which need not be 
large — occurring at the instant the discharge valves open. 

The trouble is that, in attaining compactness, simplicity, and 
moderate first cost, the power is not applied in simple, direct- 
acting pumps to the best advantage. As there is a constant 
load, but no fly-wheel to equalize the power, steam must be ad- 
mitted at full pressure throughout the entire stroke; otherwise 
the piston would be unable to reverse, and would come to a 
standstill. Such a pump must work practically without cut-off, 
and therefore a cylinderful of steam, nearly at initial pressure, is 
exhausted at each stroke. In some pumps the terminal pressure 
is quite as high as the initial. A duplex, non-compound pump, 
having a positive valve motion, may at times be even a more 
extravagant steam-consumer than a single-cylinder pump, since 
one piston may reach the end of its stroke before the other is 
ready to reverse its valve. In such case the momentum of the 
incoming steam fills the cylinder at initial pressure at the moment 
of exhaust. 

For steam-driven pumps there are several ways of improving 
these conditions : 

1. The adoption of compound or triple expansion cylinders. 
This type is suitable for the larger sizes of pump, and its use is 
increasing for mines whose depth and quantity of water warrant 
the higher first cost. The space occupied is but little greater than 
for simple pumps of the same capacity, and satisfactory results are 
obtained when they work under proper conditions and with 
sufficient initial pressure. 



252 COMPRESSED AIR PLANT FOR MINES 

2. While retaining the tandem form, a fly-wheel may be intro- 
duced, driven from the cross-head or from the steam-cylinder 
connecting-rod. This is a reversion to a type of pump long ago 
discarded for general service in this country, in favor of the 
simpler but less efficient form with no rotating parts. Although 
such a pump occupies much more space and its first cost is in- 
creased, there can be no doubt as to the advantages of being able 
to use the steam expansively, without the necessity of compound- 
ing. A large number of pumps of this description are now em- 
ployed in mines; many of the Riedler pattern and some of less 
elaborate and expensive design, such as the Prescott and others, 
in which an early cut-off — at one-quarter oj even one-eighth 
stroke — is satisfactorily adopted. 

Notwithstanding the advances made along these lines in the 
mechanical engineering of pumps and the added economy gained 
in their operation, it has been very generally assumed in the past 
that similar economies are not attainable when compressed air 
instead of steam is employed as the motive power. Yet the ad- 
vantages accruing from the utilization of compressed-air trans- 
mission in mines are marked. As the heavy losses due to radia- 
tion and the condensation of steam in pipe-lines are avoided, the 
transmission of power by compressed air may be conducted with 
a high degree of efficiency. No difficulty exists as to the disposal 
of exhaust steam underground, nor is there any danger to be ap- 
prehended from the rupture of a compressed-air pipe, while the 
bursting of a steam pipe in a shaft or in the mine workings may 
cause serious trouble. The failure to realize these advantages, 
and the unsatisfactory results obtained in most cases from com- 
pressed-air-driven pumps, are due largely to the fundamental 
differences in the behavior of steam and compressed air when 
used in a motor cylinder. In Chapter XVII reference has been 
made to the reduction of cylinder temperature accompanying 
the expansion of compressed air. The point of cut-off being the 
same, this causes lower terminal and mean pressures with air 
than with steam. In other words, at a given initial pressure and 
without reheating, a cylinderful of air develops less power. 



OPERATION OF MINE PUMPS BY COMPRESSED AIR 253 

This property of air, together with the fact that it does not 
condense, indicates clearly that steam and compressed air are not 
equally well adapted for use in an engine of the same design. It 
is not easy to understand, therefore, why mechanical engineers 
and especially pump-builders have not given more attention to 
the production of pumps properly designed for the use of com- 
pressed air. Few, if any, other branches of motor-engine prac- 
tice have been so neglected. Lack of information among- users 
of compressed air is responsible in part; in addition to which it is 
not generally realized that relatively unimportant modifications, 
at small cost, would produce much better results. Users of the 
ordinary steam pump have become accustomed to its low econ- 
omy, and, because it is strong and serviceable, it is apt to be ac- 
cepted without question when compressed air is used instead of 
steam. But in applying compressed air to the inefficient single- 
cylinder pump, as usually designed for steam, the net result is no 
better, and may be even worse, than that obtained from steam. 
The clearance spaces are large and, as the air is admitted to the 
cylinder throughout full stroke, it is used in a wasteful manner. 
Moreover, the stroke is often shortened by imperfections in the 
valve action. 

Another unfavorable feature of mine pumps driven by com- 
pressed air is the frequently improper selection of the cylinder 
proportions and arrangement of the plant. In mines having a 
number of levels the pumps are distributed according to varying 
requirements as to height of lift and quantity of water to be raised. 
The lowermost pump may have to work under a heavy head; 
others under a head of only 106 or 200 feet. As all are usually 
operated from the same pipe line arid under a common air press- 
ure, it is clear that the dissimilarity of working conditions must 
be met by proportioning the water and power ends of "each pump 
according to the work to be done. But, through error or care- 
lessness, the power end is often badly out of proportion, the tend- 
ency being to err on the side of furnishing too much power. 
The steam (or air) cylinder may be of such size as to require a 
pressure of only 30 or 40 lbs. per sq. in., while the pipe-line press- 



2 54 COMPRESSED AIR PLANT FOR MINES 

lire is 70 or 80 lbs., as usual with mine compressor plants. So it 
often happens that the deepest pump in the mine is the only one 
operating under a proper pressure. The cylinders of the others, 
even if running under throttle, are filled with air at full pressure 
when exhaust takes place.* 

The difficulty with common direct-acting pumps is thus two- 
fold : the air is used without expansion, and the pressure is often 
higher than is necessary. Recognizing, however, the convenience 
with which the inexpensive, ready-made single-cylinder pumps 
may be installed, and that in many cases efficiency of operation is 
really a secondary consideration, a few points will here be dis- 
cussed as to their employment, and the volume of air required 
for a given quantity of work. Questions relating to the 
expansive use of compressed air for pumps will be taken up 
afterward. 

Cylinder Dimensions of Simple Pumps. In calculating 
the sizes of cylinders for a simple, or single-cylinder pump, to 
work under given conditions, the dimensions of the water cylin- 
der must first be determined. There are three variables to be 
dealt with, viz: diameter, length of stroke, and number of strokes 
per minute; or the last two factors named may be combined in 
the shape of piston speed per minute. The volume of water 
to be raised being given, the cylinder dimensions may be ob- 
tained from lists of standard sizes of pumps, which would usually 
be adhered to on the ground of saving in first cost. With a given 
air pressure and head of water, the diameter of the air cylinder 
obviously depends upon that of the water cylinder. The follow- 
ing relation between the two has been determined by Mr. William 
Cox : f "Area of air cylinder is to area of water cylinder as half the 
head is to the air pressure." By the same writer a ready reference 
table has been constructed, covering the air pressures generally 
used for common, direct-acting pumps: 

* Some suggestive remarks on this subject are made by Frank Richards, 
"Compressed Air," pp. 171-172. 

f Compressed Air Magazine, Feb., 1899, p. 583. (By permission.) 



OPERATION OF MINE PUMPS BY COMPRESSED AIR 



255 



Table XXVIII 

Ratios of Diameter of Air Cylinder to Diameter of 

Water Cylinder 









Air Pressure, Pounds. 






Head in Feet. 


















20 


25 


3° 


35 


40 


45 


50 


50 


1. 12 


1. 00 


0.91 


0.84 


0.79 


0.74 


'0.71 


100 


1.58 


1. 41 


1.29 


1.20 


1. 12 


1.05 


1. 00 


125 


i-77 


i-58 


i-45 


i-34 


r - 2 5 


1. 18 


1. 12 


*5° 


i-94 


i-73 


i-58 


i-45 


i-37 


1.29 


1.22 


i75 


2.09 


1.87 


1.70 


i-58 


1.48 


J -39 


1.32 


200 


2.24 


2.00 


1.82 


1.69 


i-58 


i-49 


1.41 


225 


2-37 


2.12 


i-94 


1.79 


1.68 


i-58 


1.50 


250 


2.50 


2.24 


2-05 


1.90 


i-77 


1.67 


1-58 


275 


2.62 


2-35 


2.14 


1.98 


1-85 


i-75 


1.66 


300 


2-74 


2-45 


2.24 


2.07 


i-94 


1.82 


J -73 


3 2 5 


2.85 


2-55 


2-33 


2.16 


2.02 


1.90 


1.80 


35" 


2.96 


2.64 


2.42 


2.24 


2.09 


i-97 


1.87 


375 


3.06 


2-74 


2.50 


2.31 


2.16 


2.04 


i-94 


400 


3.16 


2-83 


2.58 


2-39 


2.23 


2. 11 


2.00 


425 


3.26 


2.92 


2.66 


2.46 


2.30 


2.17 


2.06 


45° 


3-35 


3.00 


2-74 


2-53 


2-37 


2.24 


2.12 


475 


3-44 


3.08 


2.82 


2.60 


2-44 


2.30 


2.18 


500 


3-53 


3.16 


2.89 


2.67 


2.50 


2.36 


2.24 



Ratios for intermediate heads and pressures may be obtained 
by interpolation. 

In this table the unit diameter of water cylinder is taken as one 
inch. Diameters of air cylinders, as calculated, will be in deci- 
mals, and often of odd sizes not occurring in practice. After 
determining the exact diameter, the nearest standard diameter of 
cylinder would be chosen and the air pressure and piston speed 
adjusted accordingly. 

Volume of Air for Pumps Working without Expansion. To 
determine the volume of free air required to operate a direct- 
acting, single-cylinder pump, working without cut-off, the formula 
here given will be found convenient:* 

V= 0.093 W 2 — — — , in which: 

V = volume of free air in cubic feet per minute. 

h =head in feet under which the pump is to work. 

* Ibid., p. 581. 



2^6 



COMPRESSED AIR PLANT FOR MIXES 



G = gallons of water to be raised per minute. 
P = receiver gauge pressure of air to be used. 
W 2 = volume of free air corresponding to one cubic foot at the 
given pressure, P. 
In this formula, which is based on a piston speed of ioo feet 
per minute, fifteen per cent, has been added to the volume of air 
to cover losses. The following table, giving values of W a and 
0.093 W 2 for different pressures, may be used in connection with 
the formula : 

Table XXIX 



Air Pressure P. in Pounds. 



Wo 



O.O93 W 2 



20 

2 5 
3° 
35 
40 

45 
5° 
55 
60 

65 

7° 
75 
80 

85 
90 



2.02 
2.36 
2.70 
3-°4 
3-3* 
3-72 
4.06 
4.40 

4-74 
5.08 

5-42 

5-76 
6.10 

6-44 
6.78 



0.18786 
0.21948 
0.25110 
0.28272 
°-3!434 

0-34596 
0-37758 

0.40920 

0.44082 

0.47244 

0.50406 

0.53568 

0.56730 
0.59890 
0-63054 

0.66216 



For example, let it be required to find the volume of free air 
per minute required to raise 200 gals, of water to a height of 
150 ft., the gauge pressure being 30 lbs. From the table, 
0.093 ^ 2 ' corresponding to 30 lbs. =0.2827 ; hence, 

200 X 1 ^o 



V = 0.282jX 



282.7 cu. ft. free air. 



The horse-power may be calculated from Table XXX, in 
which the mean pressures per stroke (from Table VII), for the 
different terminal pressures, are given in the second column, and 
the calculated horse-powers per cubic foot of free a.ir used, in the 
third column : 



OPERATION OF MINE PUMPS BY COMPRESSED AIR 



257 



Table XXX 



Terminal Pressure, Pounds. 


Mean Pressure per Stroke. 


Horse- Power per Cubic Foot 
Free Air. 


20 


14.40 


0.0628 


25 
3° 


17.01 
19.40 


0.0743 
0.0847 


35 


21.60 


0.0943 


40 


23.66 


0.1033 


45 


2 5-59 


0.1117 


5o 


2 7-39 


0.1196 


55 


29.11 


0.1270 


60 
65 
70 


3°-75 
3 2 -3 2 
33 - 8 3 


0.1340 
0.1406 
0.1468 


75 
80 


35-27 
36.64 


0.1527 
0-1583 



As the horse-power corresponding to a given terminal press- 
ure does not increase in constant ratio with the initial air press- 
ure, it follows that the higher pressures are not so economical 
for simple pumps as low pressures. Expressed in another way, 
the work of compression decreases with the air pressure, and there- 
fore the useful work done in a pump using air at full pressure is 
greater at low pressures and its efficiency is increased. Thus, in 
the example given above, the horse-power developed in using the 
282.7 cu - ft- °f f ree an *> at a pressure of 30 lbs., is : 

282.7X0.0847 = 23.94 h.-p. 

If the air pressure employed were 50 lbs., the cu. ft. of free 
air would be 245.52 and the corresponding h.-p., 29.36, the 
added power cost being 5.42 h.-p. It may be stated that the 
difference in favor of the lower air pressure is offset in part by the 
fact that, at the higher pressure, a pump with a smaller power 
cylinder will do the same work, thus saving in the first cost. 

But the low pressures thus shown to be suitable for simple 
pumps would not serve for machine drills, which must be con- 
sidered first, as they are in nearly all cases the chief users of com- 
pressed air in mines and quarries. To secure the best results 
from the pumps, a separate, low-pressure compressor would be 
required, a provision which is usually out of the question. Since 



17 



2 5 8 



COMPRESSED AIR PLANT FOR MINES 



it is generally necessary to use high-pressure air, at, say, eighty 
or ninety pounds gauge, the air must either be wire-drawn into 
the pump cylinder or else reduced to the required pressure before 
being delivered to the pump. 

In the first case, the results as to volumes of air used, as given 
in the preceding discussion and tables, must be modified by in- 
troducing a factor of increase, based on the ratio which the press- 
ure to be used in the pump bears to the pressure carried in the air 
main. Edward A. Rix furnishes a table,* part of which is 
abstracted in Table XXXI. It shows the volumes of free air 
theoretically required for a unit of 10,000 ft. -gals, of work 
( = 83,000 ft. -lbs. or 2.5 h.-p.), at different air pressures, together 
with the actual air consumption and horse-powers, all referred to a 
standard receiver pressure of 90 lbs. 

Table XXXI 



Gauge 
Pressure, 
Pounds. 


Ratio of 
Compres- 
sion, Re- 
ferred to 90 

Pounds. 


Cubic Feet 
of Air Cal- 
culated from 

Cox's 
\ ^Formula. 


Factor of In- 
crease for 
Wire- Draw- 
ing from 90 
Pounds. 


Increased 

Volume, 

Cubic Feet. 


Actual 
Horse- 
Power 
at 90 Pounds. 


Efficiency on 
Basis of 2.5 

Horse- Power 
Theoretical. 


20 


3- 


113 


1.26 


142 


28.6 


9 


2 5 


2.6 


108 


1.22 


125 


2 5- 


10 


3° 


2 -3 


97 


1. 19 


115 


23- 


11 


35 


2.1 


93 


1. 17 


108 


21.5 


11. 6 


40 


1.9 


89 


1. 14 


102 


20.5 


12.2 


45 


i-7 


87 


I. 12 


97 


19.7 


12.7 


5o 


1.6 


85 


I. II 


93 


19. 


I3- 1 


55 


i-5 


82 


I.09 


89 


18.2 


13-7 


60 


1.4 


80 


I.07 


86 


17.4 


14-3 


65 


I-3 1 


79 


I.06 


84 


16.8 


14.9 


70 


1.24 


78 


I.05 


82 


16.4 


15-3 


75 


1. 17 


77 


I.04 


80 


16. 


15.6 


80 


1.1 


76 


I.03 


78 


15-6 


16. 


85 


1-05 


75 


I.02 


76 


15.2 


16.4 


90 


1.0 


74 


I.O 


74 


14.8 


16.9 



The factors in column 4 are assumed as about 70 per cent, of 
the ratios of the absolute temperatures due to expansion of the 
air from 90 lbs., to the air pressures in column 1. They may be 
taken to apply when the length of air main from the compressor 

* Transactions Technical Society of the Pacific Coast, Aug. 3d, 1900. 



OPERATION OF MINE PUMPS BY COMPRESSED AIR 259 

to the pump is moderate, as in carrying the air to a pump situated 
at the bottom of an ordinary shaft. The showing is a poor one, 
but the unfavorable working conditions, as to the type of pump 
and mode of using the air, must be taken into account. 

In the second case, the normal air pressure carried in the 
mine (say, ninety pounds) may be reduced to a suitable pump 
pressure by placing a reducing valve in the air main. The in- 
crease of volume thus produced will be accompanied by a con- 
siderable drop in temperature, so that the full increase is 
not realized. Part of the lost heat will be regained by friction, 
and from external sources if there be any considerable length of 
pipe between the reducing valve and pump; but the efficiency 
will be materially increased if the cold, partly expanded air be 
passed first into an underground receiver and thence to the pump. 
This arrangement has been satisfactorily adopted, for example, in 
the case referred to in Chapter XVIII. An adjustable spring- 
reducing valve is set to furnish any desired pressure below that 
in the main. That is, the volume of air allowed to pass is such as 
to maintain automatically a certain difference in pressure be- 
tween that in the main and the pipe leading to the second receiver. 
The latter serves three purposes: (1) if it be of ample size or of 
the tubular type the air will regain nearly, if not quite, its normal 
temperature; (2) much of the entrained moisture will bedeposited, 
and trouble from freezing avoided ; and (3) the receiver, if placed 
near the pump, will minimize the pulsations and equalize the air 
pressure. 

In the particular instance to which reference is here made, 
two underground receivers were installed 300 feet apart, the re- 
ducing valve being put in the main just above the first receiver. 
This arrangement not only caused a very complete deposition 
of the moisture, but the air entirely recovered its normal 
temperature by the time it left the second receiver on its way to 
the pump. The main air pressure was 85 lbs., and at the pump 
about 45 lbs. Indicator diagrams showed 128.5 horse-power de- 
veloped by the compressor and 16.45 horse-power at the pump, 
or an efficiency of 12.5 per cent.; thus agreeing quite closely with 



260 COMPRESSED AIR PLANT FOR MIXES 

the figures in Table XXXI. Subsequently, by compounding one 
of the pumps, using 62 lbs. initial pressure in the high-pressure 
cylinder and admitting some live air to the intermediate pipe be- 
tween the cylinders, the efficiency was raised to 25.9 per cent. 
This must be considered a fairly satisfactory performance for a 
pump not specially designed for its work. 

By adopting stage compression or by reheating, or both, the 
total efficiency can of course be increased considerably beyond 
the efficiencies shown in the table. Mr. Rix states, in his article 
previously mentioned, that by actual test of a number of simple 
pumps he has found their work to be approximately 135 
ft. -galls, per cu. ft. of free air. For stage compression the 
efficiency is increased by 15 per cent, (giving, say, 155 ft. -gals.), 
and by reheating the 135 ft. -gals, is increased by the ratio of 
the absolute temperatures under which the pump works, without 
deducting the small cost of reheating. 

Prevention of Freezing of Moisture. Though this subject 
has already been discussed at some length, several additional 
points may be noted in connection with pumping. Some benefit 
may be derived by leading a jet of water from the pump column 
into the air pipe, just before reaching the pump. A very small 
quantity of water will suffice to prevent an excessive drop in the 
temperature of the exhaust. A better way is to tap a one-quarter- 
inch pipe into the column pipe, draw down the end of this pipe to, 
say, one thirty-second of an inch and insert the nozzle so formed 
into the exhaust port. The author has observed the plan of 
carrying a small steam jet close to the exhaust port; but it is 
obvious that this is feasible only when steam is used near-by for 
some other purpose. Moreover, steam so applied is utilized much 
less perfectly than when used in a cylinder jacket. If steam be 
available, a little may be injected into the feed air pipe near the 
pump. An intimate mixture between the steam and air is thus 
produced, and in condensing the latent heat of the steam is given 
up. If water at 212 F. be injected, each pound in cooling down 
to 32 F. will give up 180 thermal units. But with steam at the 
same initial temperature, each pound in condensing gives up 966 



OPERATION OF MINE PUMPS BY COMPRESSED AIR 26 1 

thermal units, in addition to the 180 units imparted in cooling to 
32 °. Still another mode of preventing freezing is to warm the 
compressed air by passing it through a coil of pipe, placed in an 
enlarged section of the water column, or else in the pump-suction 
pipe. 

Compressed-Air-driven Compound Pumps. It is a commonly 
held idea that if compressed air be used for operating compound, 
direct-acting pumps, it should be employed like steam, with a 
cut-off in each cylinder. The resulting drop in cylinder tempera- 
ture would be obviously less than that caused in a single cylinder 
by the same ratio of expansion from a given initial pressure. 
But in aiming thus to attain a higher efficiency, by adopting the 
largest possible range of expansion, very low cylinder temperatures 
would still be produced. The loss of heat takes place chiefly 
within the cylinder, instead of in, and just outside of, the exhaust 
port, as is the case with pumps working at full pressure. Fur- 
thermore, though the same total fall of temperature occurs in 
either case, when the air expands within the cylinder the force 
of the exhaust is diminished by the low terminal pressure, and 
the inner portions of the ports are the more liable to be choked 
with ice. 

In order to use the air expansively the necessity for reheating 
in some form is clearly indicated, aside from any question of gain 
in economy. Various plans have been tried of warming the cylin- 
ders by the application of external heat, such as enveloping them 
in a hot-air jacket, surrounding them by water, even heating them 
by the flames of large lamps or torches. But, aside from other 
objections to such devices, air is too poor a conductor of heat to 
render these means at all efficient. 

The mode of applying extraneous heat may be varied in 
several ways, viz: (1) Preheating the compressed air sufficiently 
to permit of a reasonably early cut-off in each cylinder, while still 
avoiding too low an initial temperature in the low-pressure cylin- 
der; (2) in addition to preheating, the air may be reheated be- 
tween the cylinders ; (3) using cold air at full pressure in the high- 
pressure cylinder and expanding into the low-pressure cylinder, 



262 COMPRESSED AIR PLANT FOR MINES 

with or without reheating; (4) using cold air at full pressure in 
both cylinders, the air being expanded between them, with the 
application of reheating. 

The first two methods are feasible when the compound pump 
is of suitable design and the heating properly applied; but there 
would be an undesirable variation in power and speed, for an 
engine necessarily working under a constant load, if the pump be 
of the usual direct-acting type, without fly-wheel. Moreover, 
under the first plan a high initial temperature would be necessary. 
If the expansion be adiabatic, from an initial pressure of, say, 
eighty pounds to atmospheric pressure and normal temperature, 
the temperature to which the air would have to be preheated is 
given by the expression: 

T'=T(^)^ or, T' = 7o +459 p7^)°'%46 F. 

Although this temperature would be rapidly lowered during 
the stroke, proper lubrication of the cylinder might be interfered 
with. The third method would avoid in part the difficulty of 
variation in power and speed, though there would still be a vari- 
able back-pressure on the high-pressure piston ; but the increase 
in volume due to clearance, and on expanding into the passages 
and intermediate pipe to the low-pressure cylinder, would con- 
siderably reduce the temperature of the air, and a large further 
drop would ensue during the work of expansion in the low-press- 
ure cylinder. Such temperature drop may be prevented, or at 
least diminished, by introducing a receiver-reheater between the 
cylinders, with material gain in efficiency. This method has fre- 
quently been adopted, and on the whole is much preferable to the 
two first mentioned. 

The fourth arrangement, however, appears to be the most 
satisfactory. As has been pointed out by E. A. Rix,* in the 
practical application of compressed air to pumps only a small 
part of the total possible work of expansion within the two cylin- 
ders can be realized, even in favorable circumstances. Never- 

* Transactions Association of Engineering Societies, 1900. Mr. Rix also 
proposes the use of three-, and even four-cylinder pumps. 



OPERATION OF MINE PUMPS BY COMPRESSED AIR 263 

theless, if properly installed and operated, it becomes perfectly 
practicable to drive a compound pump by compressed air. It is 
a much more satisfactory machine than a single-cylinder pump, 
and is capable of working with a fair degree of efficiency. This 
may be accomplished by expanding the air between the cylinders 
only, restoring the consequent loss of pressure by reheating and 
employing full pressure in both cylinders. Thus no drop of 
temperature takes place in the cylinders themselves, and the press- 
ures, back-pressures, and speed are constant. Each air card is 
practically rectangular in shape. The pressure drop between the 
cylinders may be made small ; in fact, it need not be more than is 
sufficient to give the head necessary to cause an active flow of 
air into the intermediate reheater and thence to the low-pressure 
cylinder. A drop of, say, 20 lbs. for an initial pressure of 70 
to 80 lbs. will usually answer. 

The degree of heat to be imparted by the intermediate re- 
heater, to restore the heat lost by a drop of 20 lbs., would be only 
204 F., for a final temperature of 6o° at exhaust. If the pump 
be suitably situated, an ordinary fuel-burning reheater may be 
employed; or, should this be inadmissible, the water from the 
pump-suction or column pipe may be utilized for reheating, as 
already suggested. An example of this arrangement, which has 
often been cited, is to be found in the Gwin Mine, Calaveras Co., 
California.* A Worthington compound pump, having a capacity 
of 200 gals, per minute, was installed on the 600-ft. level of the 
mine. Placed in the suction pipe of the pump is a 300-horse- 
power Wainwright heater, with corrugated copper tubes. The 
water in the pump, at a temperature of 6o° to 70 F., passes 
through the heater tubes on its way to the pump-suction valves. 
The air, on being exhausted from the high-pressure cylinder, at a 
pressure of 35 lbs., passes into the heater and through the spaces 
between the tubes. In this way, the temperature of the air is 
raised practically to that of the water and, after expanding again 
in the low-pressure cylinder, is exhausted without freezing. 
Should the sump water be foul, the heater tubes must be cleaned 

* Installed by E. A. Rix. See Engineering and Mining Journal, 1905. 



264 COMPRESSED AIR PLANT FOR MINES 

from time to time; otherwise the coating of sediment materially 
reduces their conductivity. Still better results would be obtained 
from such an installation by employing a fly-wheel pump with a 
shorter cut-off. The lower temperature could then be met 
by water-jacketing both cylinders, the jackets being supplied with 
water by a small pipe from the pump column. Though the 
quantity of heat thus restored to the expanded air is far smaller 
than that which would be derived from a fuel-burning reh eater, 
this simple device is convenient and satisfactory for under- 
ground service. 

By employing reheating in connection with properly designed 
and operated air-driven compound pumps, efficiencies of 40 to 50 
per cent, may be realized. With 3 -cylinder pumps, furnished 
with intermediate heaters, the efficiencies are still higher, reaching 
even 70 per cent. Reference has already been made to the 
economic advantages of using the Cummings system of high- 
pressure transmission for operating compressed-air pumps. 



CHAPTER XXII 

PUMPING BY THE DIRECT ACTION OF COM- 
PRESSED AIR 

The different modes of raising liquids by the direct pressure 
of air, without the intervention of a piston or other moving part, 
embody no new idea, but it is only in quite recent years that they 
have taken such shape as to render them useful for pumping on a 
large scale. Besides the fundamental considerations of cost and 
efficiency of plant, which affect alike all systems of pumping, an- 
other question becomes of prime importance in connection with 
these methods of applying compressed air, viz: the practicable 
limits of depth or head at which they will work. These limits 
depend on the gauge pressure and mode of using the air. In 
point of efficiency, several forms of plant included under this head 
are distinctly inferior to well-designed steam-driven piston and 
plunger pumps. But when operated under proper conditions 
and with expansive use of the compressed air, recent modifications 
and improvements have brought several of them to a very satis- 
factory degree of efficiency. In first cost they compare favorably 
with pumps of the usual types, and, because of their large capacity 
and low maintenance cost, all possess marked advantages for 
some kinds of service. 

There are two classes of pumps in which the principle in 
question is employed : 

i. Pneumatic-displacement pumps, using compressed air with 
or without expansion. 

2. " Air-lift " pumps, working expansively. 

Pneumatic-Displacement Pumps. These are of several kinds. 
In the type form the compressed air is caused to act directly 
upon the surface of the water contained, in a submerged closed 

265 



2 66 



COMPRESSED AIR PLANT POR MIXES 



Connection 
froni Compressor 



— Discharge 




Fig. 96. — Merrill Pneumatic Pump. 



PUMPING BY THE DIRECT ACTION OF COMPRESSED AIR 267 

chamber or tank, suitable valves being provided for controlling 
the admission of air and water. As the name implies, the water 
is displaced by the air and is discharged from the tank through 
a column pipe. There may be either one or two tanks, the column 
pipe in the latter case being common to both. With one tank, the 
flow of water from the pipe is intermittent ; with two, practically 
constant, the pair of tanks then resembling in their relation to each 
other the chambers of the ordinary steam pulsometer pump. 
Aside from the simplicity of construction and absence of moving 
parts subjected to wear, which adapt it for mining, as well as for 
general service, such as pumping from wells and other sources of 
water supply, the pneumatic-displacement pump has a distinct 
advantage for pumping chemical solutions, acids, etc., which 
would corrode the mechanism of a piston pump. It is evident, 
however, that the head or pressure under which the ordinary 
displacement pumps will work is limited absolutely by the air 
pressure employed. 

The double-chamber pump, as built by the Merrill Pneumatic 
Pump Co., will serve to illustrate details of construction and 
operation. Fig. 96 is a diagram of this pump, showing the sub- 
merged chambers, with their connections to the discharge pipe. 
Air from the compressor enters a chest through an automatic 
valve, which opens connection alternately with the two water 
chambers. The air pressure to be employed depends on the 
height of lift. Since the weight of a column of water is 0.434 lb. 
per foot of head, the height to which a given air pressure will raise 
water is equal to the gauge pressure divided by 0.434; thus, air at 

80 
80 lbs. will pump to a height of ■ = 184 ft. In practice, how- 

•434 
ever, to cover friction, leakage, absorption of air by the water, 

and to provide the necessary dynamic head for overcoming inertia 
and securing a proper speed of discharge, an additional air press- 
ure is required. In terms of volume, i cu. ft. of water will be 
displaced per cu. ft. of compressed air. One cu. ft. of air at 80 

lbs. — = 6. 3 2 cu. ft. free air. To this should be added 

15 



268 COMPRESSED AIR PLANT FOR MINES 

for losses, etc., say 20 per cent., making a total of 7.6 cu. ft. free air 

per cu. ft. of water. Taking 1 gal. of water equal to 0.134 cu. 

ft., the work done per cu. ft. of compressed air, acting against a 

184 

head of 184 ft., will be: - = 180 ft.-gals. = i<03 ft. -lbs. 

0.134X7.6 6 * * 

In some cases a larger allowance than 20 per cent, should be 
made. The actual work done in compressing 1 cu. ft. of air to 
80 lbs. gauge, by a single-stage compressor (see Table V) is 0.183 
horse-power, or 6039 ft. -lbs. ; hence, the efficiency of the pump, on 
the basis of allowance for losses assumed above, is nearly 25 
percent., which compares favorably with the efficiencies of single- 
cylinder direct-acting pumps. 

The displacement pump in its usual form works like a simple 
piston pump, in exhausting at each stroke a tankful of air practi- 
cally at gauge pressure. By employing a series of these pumps in 
a shaft, however, and using the air expansively, it is evident 
that, with a given initial pressure, the possible height of lift and 
the total efficiency of the system will greatly exceed that shown 
above.* This can be done by a suitable valve control, by which 
the air is expanded from the lowermost tank to the one next 
above, and so on, for smaller and smaller lifts toward the top of the 
series. When the last tank is discharged, the whole system is 
occupied by expanded air, at a pressure of two or three pounds, 
which is then exhausted into the atmosphere. Air is admitted by 
the valve at intervals into the lowest tank, and the working of the 
system proceeds automatically. At 80 lbs. air pressure, water 
can thus be raised to a height of about 330 ft., instead of 184 ft., 
as in the preceding example, and at an efficiency of about 40 
per cent. 

Another displacement pump is the Latta-Martin, designed 
chiefly for raising large volumes of water under low heads; 
though it may be constructed for any desired air pressure and 
head.f- It consists of a pair of submerged cylindrical tanks, 

* This series system of tanks has been proposed by E. A. Rix, Transactions of 
the Technical Society of the Pacific Coast, Aug. 3d, 1900, p. 187. 
f Compressed Air Magazine, Jan., 1907, p. 4332. 



PUMPING BY THE DIRECT ACTION OF COMPRESSED AIR 269 

taking water through large disk valves in the bottom. On the 
tops of the tanks is placed the valve mechanism for distributing 
the air alternately into each side. This valve gear comprises a 
main and auxiliary valve, each thrown by a piston valve, similar 
to those of many single-cylinder steam pumps. The movements 
of the valves are caused by the oscillation of a pair of levers, from 
each of which is suspended a bucket filled with water and hanging 
in a housing contained within the main tank. When the pump 
is in operation, the bucket housings are alternately filled and emp- 
tied of water, so that the difference in effective weight of the 
buckets causes them to rise and fall. 

The Harris, or return-air displacement pump, made by the 
Pneumatic Engineering Co., uses the compressed air with some 
degree of expansion. There are two tanks, either submerged or 
within suction distance of the sump, each connected by a pipe 
with the compressor. The water enters by siphon action, the 
inlet, as well as the discharge valves, being placed above the tanks. 
Instead of being exhausted into the atmosphere at each stroke, 
after doing its work, the compressed air is conducted back to the 
intake of the compressor and expands behind its piston. There- 
fore, the system is a closed one, the same air being used over and 
over, in a manner similar to the operation of the Cummings 
return-air plant. The water chambers fill and discharge alter- 
nately, the admission and discharge of the air being governed by 
an automatic switch-valve, connecting the two air pipes close to 
the compressor. 

In starting, after the water in one of the tanks has been ex- 
pelled, the switch reverses and places this tank in connection with 
the compressor intake. Then, while the second tank is being dis- 
charged, the compressed air exhausted from the first returns to the 
compressor and, acting expansively upon the intake side of the 
piston, reduces by so much the power required to drive the com- 
pressor. When the pressure in the first tank has fallen suf- 
ficiently (by being in communication with the compressor intake), 
it will again fill with water. Thus, the compressor transfers the 
same body of air from one tank to the other, additional air to 



270 COMPRESSED AIR PLANT FOR MIXES 

make up for leakage being supplied through an adjustable check 
valve in the intake pipe. This valve is set to open during the 
suction period, at a negative pressure a little greater than the 
pressure required to draw water into the tanks. The switch- 
valve is operated automatically; either by a device acting at the 
intervals required to complete a cycle in both tanks, or by an 
electric make-and-break mechanism, controlled by a pressure 
gauge on the air intake. In the first case it would consist of a 
piston valve, operated by a small air cylinder, compressed air 
being admitted alternately to each side of the piston in the latter 
through an auxiliary valve. The volume of air required for a 
given size of tank may be determined in terms of revolutions of 
the compressor. 

The Harris pump has a high efficiency, say fifty-five to sixty 
per cent., and requires but little attention during its operation. 
It may be adopted for shaft pumping by installing it in several 
units, one above another, according to the total lift. 

The Halsey pneumatic pump is also made by the Pneumatic 
Engineering Co. It has a single, submerged tank, with a simple, 
automatic valve-motion, operated by a float. 

If a displacement pump be required to work in acid water, such 
as frequently occurs in mines containing pyritiferous ore, the 
pressure tanks may be lined with concrete and the other parts 
made of bronze; or the tanks may be replaced by excavations in 
the rock, adjacent to the shaft and lined with concrete or asphalt. 

Air-lift Pump. This, like the displacement pump, is a revival 
of an old principle. Since 1888, in which year Dr. Julius Pohle 
proposed its application for pumping and erected an experimental 
plant, the air-lift has attained considerable prominence. Thus 
far it has been employed chiefly for raising water from deep wells, 
as for water-supply plant, but is applicable to a limited extent 
also for pumping in shafts and for elevating finely divided pulpy 
material mixed with water, such as the slimes and sands of cya- 
nide and concentration mills. 

The pump consists essentially of two pipes: a large column 
or deliver}- pipe and a relatively small air pipe, connected with the 



PUMPING BY THE DIRECT ACTION OF COMPRESSED AIR 271 



compressor receiver. A diagram of the typical form of the ap- 
paratus is shown in Fig. 97. The delivery pipe, open at both 
ends, is submerged in the water to a depth proportionate to, but 
always greater than, the 
height to which the water 
is to be raised. The com- 
pressed-air supply pipe 
passes down to a point 
near the bottom, and ter- 
minates in a nozzle, which, 
directed vertically upward 
by a return bend, is insert- 
ed in the lower open end or 
foot-piece of the delivery 
pipe. (Modifications of 
this arrangement are noted 
hereafter.) 

In some respects the 
operation of the air-lift 
pump is the reverse in 
principle of the method of 
compressing air by the di- 
rect action of falling water. 
As the compressed air 
leaves the small pipe it ex- 
pands and, if the discharge 
pipe is of small diameter, 
tends to form piston-like 
layers, which rise rapidly, 
alternating with masses of 
water. This is readily 
shown by experimenting 
with glass tubes. But if 
the discharge pipe be of 
large diameter, the air 

Should be admitted through FlG< 97 ._ D iagram of Pohle Air-Lift Pump. 




■-*■ 11 



272 COMPRESSED AIR PLANT FOR MINES 

a series of ports or nozzles, resulting in a dissemination through 
the rising water of small masses of air or bubbles. The water is 
raised chiefly by the buoyancy of the air; or, expressed differently, 
by the aeration of the column of water, which causes a reduction in 
its specific gravity. The action of the pump is due in a small degree 
only to the expansive force and vis viva of the compressed air. It is 
obvious that, before the air is turned on, the water stands at the 
same level inside and outside of the delivery pipe. On entering 
the foot-piece, the air is under a pressure due to the weight of the 
rising column of mixed air and water. As the bubbles of air rise, 
in forcing the water upward, they expand with the decrease in 
head; so that, on reaching the top of the column, the com- 
pression is that due only to the weight or pressure of the small 
quantity of water about to issue from the pipe. Thus, the air 
leaves the pump column at a pressure but little above atmos- 
pheric pressure. The initial air pressure required depends on the 
pressure due to head, measured from the nozzle or air ports 
to the surface of the water. If the pressure be too high loss of 
work ensues at the compressor. Should the delivery pipe be 
too deeply submerged, in proportion to the net height of lift, an 
uneconomically high pressure will be required to force the air into 
the foot-piece; and, with an insufficient submergence a larger 
quantity of air will be necessary to produce the velocity of delivery. 

Referring to Fig. 97, let : 
hi = depth to which the delivery-pipe foot-piece is sunk below the 
normal level of the water, before pumping begins, or when 
the water is at rest ; 
h 2 — height at which the water stands when the pump is in opera- 
tion; 
H = height of the column of mixed air and water, measured from 

the air inlet to the point of discharge; 
L=net height of lift = H — h 2 . 

The compressed air enters the foot-piece at a pressure, P', 
corresponding to the head, h 2 ; or, h 2 X pressure per foot of 
hydraulic head =0.434 h 2 . Assuming that the water rises in piston- 
like masses — as would be the case with a single air nozzle and a 



PUMPING BY THE DIRECT ACTION OF COMPRESSED AIR 273 

delivery pipe of small diameter — the sum of the lengths of these 
masses in the column H must be theoretically equal to the outside 
solid column of water, h 2 . (The weight of the compressed air 
contained in the column may be neglected.) But, to overcome 
the f rictional resistance and produce flow, the head h 2 must be 
greater. Under ordinary working conditions, the net height of 
lift, L, is found to be from 0.5 h 2 io say 0.65 h 2 . Taking the second 

L 

value and transposing: h 2 =——; and by substituting in the 

L 

expression for the value of P', as above: P' =0.434 ——- = 0.67 L. 

50 
If, for example, L be 50 ft., P' = 33.5 lbs., and h 2 = —— = f jj ft. 

Since the air in the column H is divided into small masses, 
surrounded by water, its expansion during the upward flow may 
be assumed to be isothermal. If P' be its initial pressure, the 

/P'\ 
mean pressure for the entire lift = PxNap. log. I — KPandP' 

being absolute pressures. In the above example, taking P as 
15 lbs., P' = 33.5 + 15 =48.5 lbs., whence, the mean pressure = 
17.5 lbs. gauge. 

For starting the pump, the air pressure must be sufficient to 
overcome the normal static head, h iy but, when the flow has begun, 
the pressure required falls to that corresponding to h 2 . Though 
this difference in pressure (h T — h 2 ) may be considerable, it is 
readily met by temporarily speeding up the compressor. To mini- 
mize fluctuations between h x and h 2 , the top of the well or sump 
should be extended laterally, in order to furnish a large horizon- 
tal area of water, the level of which would be but little affected 
by stoppages or by variations in air pressure and delivery. The 
throttle valve in the air pipe may be regulated by a float on the 
surface of the water. Care should be taken in the design of the 
foot-piece and in properly proportioning the air pressure to the 
submergence and net lift. Otherwise, air may leak back into the 
sump or outside column of water; and, if this becomes aerated, 
18 



274 COMPRESSED AIR PLANT FOR MINES 

much more power and a larger volume of air will be required to 
keep the pump in operation. In such case the efficiency is greatly 
decreased. 

Since 1889 many experiments by competent engineers have 
been made on the air-lift pump. Among the first were those of 
B. M. Randall and H. C. Behr, on a sixty-foot well, with a stage 
compressor. A summary of these tests is given by E. A. Rix, in 
the Transactions of the Technical Society of the Pacific Coast, Aug. 
3d, 1900, p. 206. In 1894 a series of tests were made at De Kalb, 
111.,* and in 1893 and again in 1896 on four pumps at Rockford, 
Ill.t 

The last-named were carefully carried out and the results 
compared in tabulated form. The heights of lift above water- 
level were 66.5, 90, and 91.5 ft., the air pressure being 76 lbs. 
gauge and the submersion, 225 ft. Both air pressure and depth 
of submersion appear to have been unnecessarily great. With a 
compressor of i24-h.-p., the net work done was 24-h.-p., or 
an efficiency of about 20 per cent. With 600 cu. ft. free air per 
minute, 200 cu. ft. of water were pumped, or 3 air to 1 water. 
The sizes of piping used were: delivery pipes, 4 in., 5 in., and 6 J 
in., with air pipes from ij to 2 J in. In several of these tests 
the air pipe terminated in a f-inch nozzle. The plan was also 
tried of closing the lower end of the air pipe and discharging the 
air through slot-shaped perforations in the sides near the bottom; 
but the results were inferior to those obtained from the single- 
nozzle opening. Possibly better work would have been done by 
some different arrangement or size of slots; for large pipes and 
volumes of water, at least, the single nozzle has not been found 
satisfactory. 

E. E. Johnson gives a table of the performance of the air-lift 
pump, including consumption of power and theoretical and total 
efficiencies for different height of lift, J from which Table XXXII 
is abstracted : 

. * Engineering News, July 12 th, 1894. 
t Ibid., March 4th, 1897. 
J Ibid., April 22d, 1897. 



PUMPING BY THE DIRECT ACTION OF COMPRESSED AIR 275 



Table XXXII 







Theoretical Horse-Power. 


Efficiency of Air-Lift. 


Lift In. 


. 

O CD 

§§ 

US 

<u a 

o«i 

M 
OO 
£-1 


To Deliver One Cubic Foot 
of Air per Minute. 


Theoretical 


Total Efficiency 

from Power 

Applied to 

Water Del'd. 


(/] 

V 
u 

a. . 

■as 

c ^ 
O 


T3 
0) 

CD 
CD 

fa 


13 
£ 

u 

CD 
O 


CD 

be 
a 

en 
6 


0* 
< 


Cd£ 

■£ o- 
og 
"> 


bfi.2 
ca c/) 
+j t« 
op g 
6 a 

££ 

Ho 

U 


&2 c 

•— ^ 


si- 

W CD 

6 0. 

£ S 

Ho 
U 


fry c " 
SPg.2 

C/3 oU 


5 
10 

15 
20 

25 
30 

35 
40 

45 

5o 

55 
60 

65 
70 

75 
80 

85 
90 

95 
100 
no 
120 
130 


n-54 
23.09 

34-63 
46.20 

57-75 
69.31 
80.86 
92.41 
103.90 

n5-5o 
127.00 

138.60 

150.10 

161.70 

173-30 
184.80 
196.30 
207.90 
219.40 
230.90 
254.10 
277.20 
300.40 


.O2185 

-04363 

-O6545 

.08727 

.IO9 

.I309I 

-1527 

-17454 

.I963 

.2l8l8 

.24 

.26l8l 

.2836 

-30545 

-3273 

-3491 

-37 

-3927 

-4145 

-43636 

.48 

-5236 

-5675 


.02514 
.O5586 
-09I05 
.12994 
.17191 
.21678 
.26445 

-3 J 375 
-36368 
.41848 
.47112 

-52855 
.58612 

.64812 
-70952 
-76843 
-83039 
.89444 
.96164 

1.0243 

1. 162 

1. 301 

1-443 


.02572 
.05992 
.0962 

-I39I 
.1897 
.2370 

-2915 
-3489 
-4085 
.4722 
-5366 
.6051 

■6734 
.748 
.823 
.898 
-976 
i-°55 

I - I 37 
1.247 

1-394 

i-57i 
i-755 


.0263 

.064 

.1015 

.1483 

.2004 

- 2 573 
-3187 
-3842 

-4535 

.5261 

.6023 

.6818 

.7608 

.8483 

.9380 

1. 0291 

1.1231 

1. 2176 

1. 3148 

1.4171 

1.626 

1. 841 

2.068 


-87 
-78 
.72 

-675 
-635 
-603 

-577 
-557 
-540 
.522 

-51 

-495 

-483 

.471 

.462 

-455 
-446 

-439 
-43 1 
.428 

-413 

.402 

-394 


.848 
.728 
.687 
.627 

-575 

-548 

-52 

.502 

.482 

-464 

-447 

-43 2 

.422 

.408 

-398 

-39 

-38 

-373 
.368 

-352 
-346 
■333 
-3 2 4 


-83 

.684 
.648 

-59 

-545 

-508 

.478 
-455 
-433 
-415 
.40 

-384 

-372 

-36 

-35 

-343 

■33 1 

-324 

-3 J 5 

-308 

.296 

-285 

-275 


-623 
-546 
-515 

-47 
-432 

.412 

-39 

-376 

.362 

-348 

-336 

-324 

.316 

-307 
.299 

.292 

.285 

.28 

.276 

.264 

.26 

-25 

-243 




■497 
.41 

•389 
•354 
■327 

305 
287 

273 
260 
249 

24 
231 

223 
216 

210 
206 
198 
194 
189 
185 
177 
171 

165 



These figures represent the work of well-proportioned plant, as 
to depth of submergence and air pressure. It is shown clearly 
that the efficiency of the air-lift falls off rapidly as the air pressure 
and height of lift increase. The higher efficiencies are naturally 
obtained from stage compression. In general it may be stated 
that, under normal conditions and with small lifts, efficiencies 
of from 30 to 35 per cent, are readily obtainable, and may rise to 
45 or 50 per cent., with proper air pressures and ratios of sub- 
mergence to height of lift. 



276 COMPRESSED AIR PLANT FOR MINES 

In 1906 several tests were made at Wandsworth, England, 
on a modified Pohle air-lift, with a delivery pipe of increasing 
diameter toward the top. The total height of the delivery pipe 
was 580 ft., of which 324 ft. were submerged, the net lift thus be- 
ing 256 ft. In this case the distance h x — h 2 was 69 ft., air pressure 
135 lbs., ratio of volume of free air used to water discharged, 5.8 
and 5.6 . 1. The total efficiency was 36 per cent. In view of the 
conditions this is an excellent showing and indicates an advantage 
in using a tapering column pipe. 

The following results of a test made on a 300-ft. well will 
further illustrate this subject:* 

Elevation of discharge above mouth of well 85 ft. 

Depth to water-level during operation of pump 44 " 

Net lift, water-level to point of discharge 129 " 

Submergence of delivery pipe 248 " 

Air admitted to delivery pipe 5 ft. above inlet end. 

Diameter of delivery pipe 3.5 ins. 

" " air pipe 1.25 " 

Volume of water delivered per minute 82.5 gals. 

" " free air used per minute 81.8 cu. ft. 

Gauge pressure of air 107 lbs. 

Consumption of free air per cu. ft. of water 7.44 cu. ft. 

Horse-power consumed by compressor 12.1 " " 

Total efficiency 22.3 % 

A number of calculated values for air-lift pumps are included 
in Table XXXIII. 

The question of relative sizes of air and delivery pipes has not 
yet been satisfactorily answered. While there are many varia- 
tions in practice, it is probable that ratios of diameter ranging 
from 1 : 2 up to 1 : 2 J or 3 will be found suitable. The absolute 
diameters of the pipes are determined on the basis of frictional 
loss caused by the flow of the air and water. A water velocity of 
250 to 300 feet per minute may be assigned for the delivery pipe. 
The friction losses in air pipes have been discussed in Chapter 
XVI. It should be added that when the water is delivered at a 

* G. C. H. Friedrich, Trans. Ohio Soc. 0} Mech., Elec, and Steam Engrs., 1906. 



PUMPING BY THE DIRECT ACTION OF COMPRESSED AIR 277 

distance from the pump, the additional frictional resistance must 
be determined, and the air pressure and submergence corre- 
spondingly increased. Reference may be made in this connection 
to a paper by H. T. Abrams, in Compressed Air Magazine, Aug., 
1906, p. 4135. 

Table XXXIII 





Volume of Air per 


Submergence, at 




Horse-Power per 


Lift, Feet. 


Cubic Foot 


Sixty per Cent of 
Total Height of 


Air Pressure. 


Gallon Water 




of Water. 




per Minute. 






Delivery Pipe. 






25 


2 


38 


17 


0.0184 


50 


3 


75 


33 


0.0426 


75 


4-5 


IT 3 


49 


0.0828 


TOO 


6 


150 


65 


0. 1320 


125 


7-5 


188 


82 


0.1910 


I50 


9 


225 


98 


0.2544 


175 


10.5 


263 


ii5 


0.3150 


200 


12 


300 


130 


0.3808 



Among the most complete and valuable recent tests of the air- 
lift pump are those made in 1907 by Messrs. Henderson and Wil- 
son at the two 200-stamp mills of the Angelo and Cason mines, of 
the East Rand Proprietary Mines, Limited, South Africa.* At 
these mills both slimes and sands are raised to the settling tanks 
by air-lift pumps, instead of the usual tailings-pumps and wheels. 
The delivery pipes used in the 19 tests recorded were of two kinds, 
viz: 10- to 16-in. pipes of constant diameter, and several pipes 
increasing in diameter from 12 and 14 ins. at the bottom to 14 
and 16 ins. at the top. These pipes did not taper uniformly, as 
this is impracticable; but, for a length of 35 ft. above the air inlet, 
were lined with one inch of wood, which served incidentally to pro- 
tect the metal from the scouring action of the mixture of sands or 
slimes and water. 

The foot-piece used in the earlier tests was flared out and 
closed at the bottom, the water and pulp being admitted through 
4 large ports, 2 J ft. below the air inlet and having a combined 
area of about 200 sq. ins. The air inlet was a single opening, 4 

* The Engineer (London), Jan. ioth, 1908, p. 26. 



2 7 8 



COMPRESSED AIR PLANT FOR MIXES 



ins. diameter. For the later tests, the foot-piece was open at 
the bottom and modified by flaring it out to double the diameter 
of the column pipe, so as to increase gradually the velocity of in- 
flow. And, instead of a single air inlet, a ring of twelve holes, one 
inch square, admitted the air: these holes being cast in an an- 
nular recess a little larger in diameter than the throat of the foot- 
piece. This design gave materially higher efficiencies than that 
first used, as shown by the following table, which, though pre- 
senting the details of only four of the tests made, indicates in 
general the results obtained. 

Table XXXIV 



Number and size of delivery- 



pipes 



~ 



Submersion in feet 

Lift in feet 

Ratio of submersion to lift 
Gauge pressure of air ; lbs. . . . 

Kind of foot-piece 

Throat diameter of foot-piece 



Free air, cu. ft. per minute... 

" " per cu. ft. of slimes . . 
Cu. ft. of slimes per minute . . 
Throat velocity, cu. ft. per 

second 

Theoretical horse-power in 

pulp raised 

' Horse-power per cu. ft. free 

air compressed 

Air horse-power i c v . - : 



Two io-in. 


Two io-in. 


One 16-in.. 
decreasing 
to 14 ins. 


One 14-in., 
decreasing 
to 12 ins. 


32.75 

J2-5 

1.009 to z 

15 

Original 

to ins. 


:: -75 
29-5 
1. 2 1 to 1 

16 

Original 

10 ins. 


; "- 75 
27.5 

1.372 to 1 

17 
Modined 
13J ins. 


48. 85 

27.09 

1.77 to 1 

22 
Modified 
iij ins. 


2 , >6 


1279 


~±6 ■ B 


M 


310 


4.06 

3*5 


2-74 

: :: 


2.64 
320 


4-7 


-•> 


4-85 


7-39 


19-3 


17-8 


*5- 2 3 


16.6 


.048 

icS. -2 


.050 

64-74 


■053 
42.21 


54-14 



Efficiency, per cent. 



^6.1; 



In the paper from which the above data are abstracted full 
details of all the tests are given. The conditions were modified 
in the progressive tests, as to the ratio of submersion to lift, 
diameter of deliver}- pipe, and air pressure. As a basis for cal- 
culating the theoretical horse-power represented by the mixture 
of water and pulp raised, the weight of the slimes was determined 



PUMPING BY THE DIRECT ACTION OF COMPRESSED AIR 279 

to be 63.3 lbs., and of the sands, 64.56 lbs., per cu. ft. Thus, 
for the sands, this horse-power was taken to be : 

(Quantity of sands+water) X 64.56 X ft. lift _ OOIQq6X Q Xft lift 
33,000 

The term "sands" refers to the mixture of water and ore as 
crushed by the stamps, from which the " slimes " have been 
separated in the milling process. 

LanselPs Air-lift. An interesting modification of the air-lift 
pump, as applied by Mr. George Lansell to pumping water from 
a deep mine shaft in the well-known Bendigo district, of Victoria, 
Australia, may be described here. In the shaft in question water 
has been raised in a series of lifts from a depth of 1,385 feet. Fig. 
98 shows diagrammatically the arrangement of the parts for two 
of the lifts. 

The compressed air is conveyed from the receiver in a pipe, A, 
running down the shaft. The water is conducted from the tank 
or sump through a pipe, D, which first passes down the shaft a 
certain distance, depending upon the height to which the water 
is to be raised, and is then connected with an enlarged section of 
pipe, E, at the foot of the column or delivery pipe, B. Thus, 
the piping for each lift has the form of an inverted siphon, through 
the longer leg of which the water is discharged. At the lowest 
point of the siphon a short branch pipe, C, enters from the air 
main, A, the end of this branch being directed vertically upward 
into the foot-piece, E. Before the compressed air is turned on the 
water stands at the same level in the pipes D and B. The effect of 
this arrangement is similar to that produced by submerging in the 
body of water to be raised the lower part of the delivery pipe, as in 
the Pohle air-lift pump. Check valves are placed, as shown, in 
the pipes D and C, to prevent air or water from passing back into 
the air pipe or into the tank. A throttle valve is provided in the 
pipe C, for regulating the supply of air as required. The relative 
heights of the various parts are not fixed, the dimensions as shown 
on the sketch indicating substantially the proper depth of the 
inverted siphon below the tanks, and the corresponding height of 
lift; thus, from the tank at the 250-ft. level, the pipe D passes 



2 SO 



COMPRESSED AIR PLANT FOR MINES 




Fig. 98.— Diagram of Lansell's Air-Lift Pump for Mine Shafts. 



PUMPING BY THE DIRECT ACTION OF COMPRESSED AIR 28 1 

down the shaft 140 ft., to the foot of the delivery pipe which dis- 
charges at the surface. A series of lifts may thus be arranged to 
raise the water from any desired depth. The pressure of air is 
the same for all, this pressure being sixty to eighty pounds per 
square inch, or that which is ordinarily furnished for mine 
service. 



CHAPTER XXIII 

COMPRESSED AIR HAULAGE FOR MINES 

For the underground transportation of ore or coal, compressed 
air may be utilized either in locomotives or for driving stationary 
rope-haulage engines. Before taking up the subject in hand, a 
few considerations will be set forth respecting the operation of 
mine locomotives by steam and electricity as well as by com- 
pressed air. Steam locomotives are now much less frequently 
used than formerly for underground haulage, and they can be em- 
ployed only in mines where the trains are conveyed through tun- 
nels or entries directly to the surface, so that stoking may be done 
outside of the mine. Though uneconomical consumers of power, 
steam locomotives are rendered practicable in some collieries 
chiefly by the fact that the fuel is a product of the mines them- 
selves and is therefore chargeable at a low cost. Their principal 
disadvantage lies in the serious vitiation of the mine atmosphere 
caused by the discharge into the workings of the products of 
combustion. Obviously they cannot be employed in gassy or 
fiery mines. 

Electric and compressed-air locomotives divide between them 
a much broader field of operation. Both are applicable to mines 
of all kinds, whether collieries or metalliferous mines; for either 
long or short hauls, from a few hundred feet to several miles; 
they may be used underground in mines worked through shafts, 
where cars cannot be hauled through a tunnel to the surface, but 
must be hoisted on cages, and they do not vitiate the mine at- 
mosphere. For underground haulage in mines containing fire- 
damp, however, electric locomotives must be adopted with cau- 
tion. Although, by the improvements introduced in recent years, 
much has been done to prevent the occurrence of serious sparking, 

282 



COMPRESSED AIR HAULAGE FOR MINES 283 

some risk from this cause still exists; and, furthermore, the 
possibility of strong sparking, accompanied by the momentary 
development of intense heat, from short-circuiting or by reason of 
a ruptured conductor, can hardly be averted. 

Compressed-air locomotives were probably first used in the 
works of the Plymouth Cordage Co., Plymouth, Mass., about the 
year 1873; an( ^ m Great Britain, for mine haulage, in 1878, 
though these early designs were quite different from those now em- 
ployed, and not very successful. Their introduction in the United 
States proceeded very slowly for some years. Perhaps twenty 
compressed-air locomotives were built previous to 1898, but since 
then they have been applied widely for a variety of service.* 
Expressed in general terms, the plant consists of a compressor 
(usually three-stage), receiver, pipe-line, charging stations, with 
the necessary valves and one or more locomotives. The storage 
tank or tanks carried by the locomotive are charged with a suf- 
ficient volume of high-pressure air for a round-trip run of the 
maximum length required, after which the locomotive returns to 
the nearest charging station for a fresh supply of air. 

The special advantages of compressed air, as compared with 
electric haulage for mines, are : First, it may be used in collieries 
with perfect safety, in an atmosphere charged with fire-damp or 
dust, or in dry and heavily timbered workings; second, since the 
power is stored in the locomotive itself, the system presents the 
maximum degree of flexibility. The locomotives can enter all 
parts of the mine, wherever track is laid, far beyond the limit 
of the supply-pipe line, and are not, like electric locomotives, de- 
pendent upon wiring, which must accompany every foot of ad- 
vance.")* For collieries they may be used equally well for the 
haulage of trains on main lines, and for gathering and distrib- 
uting individual cars among the working places ; third, the com- 
pressed air costs little or nothing when not in actual use, and its 

* Letter to the author from the H. K. Porter Co., Pittsburg, Pa. 

fit should be noted, however, that storage battery and "cable-reel" electric 
locomotives have been introduced in a few cases, both in Europe and the United 
States. The latter has a very limited range of application and can be used for 
short branch lines only. 



284 COMPRESSED AIR PLANT FOR MINES 

full power or but a fraction of it is available at all times. During 
the unavoidable periods of idleness of the locomotives no power is 
wasted, because, though the compressor may continue in opera- 
tion at a slower speed, it is engaged in storing up power in the 
receiver and pipe-line. Incidentally the exhaust of the locomo- 
tives discharges fresh and cool air into the workings. While this 
is a minor consideration, it improves rather than injures the 
ventilation of the mine. Both electricity and compressed air 
must be looked upon merely as transmitters and distributers of 
power, depending for their production on either steam- or water- 
power as a prime mover. 

At most mines compressed-air haulage is employed only for 
underground transportation, from the stopes or breasts to the foot 
of the hoisting shaft; in other cases, where the mine is worked 
through a tunnel or adit-level, the locomotives haul trains of cars 
direct to the breaker, tipple, or ore-bins, situated on the surface. 
Occasionally, as for example, at the Homestake Mine, Lead, S. D., 
compressed-air locomotives are used for surface transportation of 
ore, from the crusher houses at the shaft mouths to the different 
stamp mills; the object being chiefly to reduce the fire risk for the 
wooden structures, into and near which the haulage tracks pass. 
For the same reasons many plants have been installed in and 
about manufacturing establishments, containing inflammable 
buildings or materials, such as lumber yards and explosives 
factories or magazines. 

Construction and Operation of the Locomotive. For mine ser- 
vice compressed-air locomotives have either one or two cylindri- 
cal storage tanks. These tanks, with the cylinders, piping, and 
other appurtenances, are mounted on a frame provided with 
springs similar to those of a steam locomotive and carried by 
4 or 6 driving wheels. The 6-wheel type is used where a 
heavier locomotive or a lighter rail requires the distribution of the 
load over a greater number of points. Fig. 99 illustrates a recent 
design of a four-wheel, single-tank locomotive, as built by the H. 
K. Porter Co. It is made in several sizes, the details of which are 
given in the first four columns of the following table. 



COMPRESSED AIR HAULAGE FOR MINES 



285 



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286 



COMPRESSED AIR PLANT FOR MINES 



In the last two columns of the above table are details of the 
type of locomotive shown in side and rear-end elevation in Fig. 
ioo. The one illustrated has 4X7-in. cylinders and is used 
on track of 2 7 -in. gauge for hauling mine cars from the under- 
ground loading chutes to the shaft stations. The operator's 
seat is detachable, so that the locomotive can be readily trans- 
ferred as required from one level to another, on a cage whose 
platform is 5 ft. long. These two sizes are suitable for general 




Fig. 99. 



service in metal mines, or for gathering cars from individual 
working places in collieries, to make up trains on main haulage- 
ways. 

A 6-wheel, double-tank locomotive, by the Baldwin Loco- 
motive Works, is shown in Fig. 101. It has the following dimen- 
sions: gauge, 3 ft.; cylinders, 11 ins. X 14 ins.; main tanks, 22 ft. 
7 ins. and 20 ft. 1 ins. X 34 ins. diameter, carrying a pressure 
of 800 lbs. ; auxiliary tank pressure, 140 lbs. ; driving wheels, 28 
ins. ; wheel-base, total, 6 ft. 6 ins. ; total weight, 39,050 lbs., all on ' 
driving wheels. Another Baldwin locomotive, of the 4-wheel 
type, with 9X 14-in. cylinders, 5-ft. 6-in. wheel-base, and weighing 
24,350 lbs., is shown in Fig. 102. These builders make a number 
of other sizes of mine locomotive, the smallest weighing 8,000 lbs., 




o 



288 



COMPRESSED AIR PLANT FOR MINES 



and having 5 Jx 10-in. cylinders; track gauge, 36 ins. ; tank press- 
ure, 900 lbs., and working pressure 170 lbs. Some of the larger 
sizes are designed for a cylinder pressure of 200 lbs. Compressed 




Fig. ioi. 




Fig. 102. 

air mine locomotives are built also by the American Locomotive 
Company. 

Where there are sharp curves in the track, as is commonly the 
case underground, the wheel-base must be short, say 4 ft. 6 ins. 
to 6 ft., for a 4- wheel engine. The height over all of the loco- 
motive depends somewhat on the conditions existing in the mine, 



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COMPRESSED AIR HAULAGE FOR MINES 



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290 



COMPRESSED AIR PLANT FOR MINES 



as to thickness of vein, head-room of the haulageways, etc., and is 
rarely more than 5 or 6 ft. — frequently less. The length varies 
greatly, mainly according to the tank capacity required, and the 
curvature of the gangways. It is usually from 10 to 15 ft. for 
the smaller sizes, up to 20 or 24 ft. for the larger, the widths 
ranging from 3 J to 6 ft. 

Table XXXVI contains the principal data of seven sizes of 
large six-wheel, double-tank locomotives, built by the H. K. Porter 




Fig. 104. 



Co. These comprise the heaviest locomotives designed for under- 
ground mine service. 

Additional details of the construction of compressed-air mine 
locomotives are exhibited in Fig. 103, containing a general plan and 
side elevation of a Baldwin locomotive, with 9 X 14-in. cylinders. 
Fig. 104 shows a half front-end elevation of the same, with half 



COMPRESSED AIR HAULAGE FOR MINES 



29I 



section through frame and left-hand storage tank; and Fig. 105, 
a half rear-end elevation, with section of left-hand tank. 

The tanks have dished or approximately hemispherical ends, 
and are built of extra heavy steel boiler plate ; the shells being 
I to J in. thick, with 1 to ij-in. heads. Ring seams are double 
riveted with lap joints; longitudinal seams being butt joints, with 
inside and outside welt strips. As the tanks are generally built 
to carry working pressures of 700 to 800 lbs. per sq. in., the longi- 




k 18J£— * 

C — u ^lJ 



Fig. 105. 

tudinal seams have 6 to 8 rows of rivets, to make a joint of not less 
than 75 per cent, of the strength of the plate. It is customary to 
test the tanks to 800 or 1,000 lbs., the factor of safety with plate 
of the usual quality being, say, 3^. This is considered sufficient, 
as there are no strains produced by expansion and contraction, as 
in a boiler. When extremely high pressures are required, tanks of 
large diameter cannot safely be employed, and are replaced by a 
set of heavy seamless steel tubes, 8 to 9 ins. diameter — for example, 
the Mannesmann tubes. Tubes of this kind, 9 ins. diameter by 



292 COMPRESSED AIR PLANT FOR MINES 

-^ in. thick, will carry working pressures of 2,000 to 2,500 lbs. per 
sq. in. A number of them are laid together, bound by belts or 
straps and then enclosed in a light sheet-iron shell, to protect them 
from wet and rust. But these high pressures are unnecessary for 
ordinary systems of mine haulage. 

From the main tanks the air passes into a small auxiliary or 
distributing reservoir and thence to the cylinders. This auxiliary 
tank is merely a section of wrought-iron pipe from 4 to 9 ins. 
diameter and 6 to 15 ft. long, with closed ends and laid alongside 
the main tank. By means of an automatic reducing valve, the 
pressure in the small reservoir is adjusted to the requirements of 
the engine. As used on the locomotives of the H. K. Porter Co., 
the reducing valve consists of a double-seated balanced valve, 
operated by a small piston. The air pressure in the auxiliary 
reservoir acts on one side of this piston and tends to close the 
valve. This action is opposed by a powerful external spring, 
which is adjusted to keep the valve open until the normal working 
pressure is reached in the auxiliary reservoir. Then the valve is 
closed by the air pressure, against the resistance of the spring. To 
provide for the case when the locomotive is using no air (as on a 
down grade or when at rest), a single-seated supplementary valve 
is placed in the pipe between the reducing valve and the loco- 
motive storage tanks. This valve is controlled by the throttle 
lever; being open when the throttle is open, otherwise closed by 
the air pressure. By thus using two valves leakage from main 
tanks to auxiliary' reservoir is avoided and a close regulation 
secured. 

The cylinder pressure adopted ranges generally from 125 to 
1 50 lbs., according to the size of cylinder and power required, thus 
being about one-quarter of the pressure in the main tank. From 
the small tank the air passes to the cylinders through a balanced 
throttle valve. This arrangement permits the maintenance of a 
constant working pressure, suited to the needs of the locomotive, 
prevents the waste of aii likely to ensue if air at full tank pressure 
were admitted to the cylinders, and makes the locomotive more 
manageable. The cylinders, moreover, need not be made so 



COMPRESSED AIR HAULAGE FOR MINES 293 

heavy as would be required for a high pressure. In starting a 
heavy load excessive slipping of the drivers is avoided, and with 
light loads the reducing valve may readily and quickly be regu- 
lated to produce any desired reduction of pressure. In the opera- 
tion of the locomotive toward the end of the haul, when the press- 
ure in the main tanks falls to that in the auxiliary tank, the 
cylinders take their air directly from the former, and the loco- 
motive will continue to run as long as the pressure remains 
sufficient. Sometimes, for long hauls, and when the cross- 
sectional dimensions or sharp curves, or both, of the haulage- 
way do not permit the use of tanks of great length or large 
diameter, a tender carrying a supplementary tank" is employed. 

For small-scale work, the air is sometimes admitted to the 
cylinders throughout nearly full stroke, and consequently, as the 
exhaust is at high pressure, the efficiency is lower than it should 
be. This practice is doubtless due to the tendency to use as 
small a motor as possible for the service required, on account of 
the limited head room and narrow, crooked gangways so common 
in mines. Better results are obtained by using a cut-off and in- 
creasing the size of the locomotive and the weight on the drivers. 
This is almost always done with large locomotives. Ample re- 
serve power is available when necessary, since full tank pressure 
can be temporarily admitted to the valve-chests in starting a 
heavy load, or in hauling on steep grades and around sharp curves. 
In using the air expansively, as can be done with properly pro- 
portioned cylinders, there should be no trouble from freezing of 
the moisture. Although the expansion will produce a low cylin- 
der temperature, yet, as the initial working pressure is so much 
higher than is employed for pumps or other compressed-air 
machinery, the expanded air becomes relatively dry, and the force 
of the exhaust is still sufficient to keep the ports clear of accumu- 
lated ice. To this end the ports should be large, straight, and 
short, though ports of ordinary proportions are quite common. If 
high-pressure air were used in the engines, both cylinders and 
pistons would have to be made excessively heavy, and any reason- 
able degree of expansion would produce a degree of cold difficult 



294 COMPRESSED AIR PLANT FOR MINES 

to deal with. The cylinders should not be lagged with non-con- 
ducting covering, as is so necessary for steam cylinders, to min- 
imize condensation. By exposing their surface to the warm 
air of the mine, some heat is absorbed. Usually the exterior 
surface of the cylinders is cast with deep corrugations, in order to 
present the largest possible superficial area to the warm sur- 
rounding air. The cylinders are provided with slide valves; 
piston valves, like those used in steam locomotives, would leak 
more because of the dryness of the air. 

On account of the cold produced by the reduction of pressure 
from the main tanks to the auxiliary reservoir, and to increase 
efficiency of operation, reheating is found to be advantageous, 
though not essential. It may be accomplished conveniently by 
applying heat to the auxiliary reservoir. If steam be available in 
the mine, a quantity of steam and hot water may be injected into 
this reservoir each time the locomotive is charged. Or, in mines 
where there is no danger from fire-damp, a small reheating ap- 
paratus for burning oil or coke may be carried on the locomotive. 
It is always desirable to warm the reducing valve from the main 
tank, as this is subjected to intense cold. In any case, when the 
air is reheated a quantity of water should be kept in the small 
tank. An incidental advantage of this arrangement is that the 
moisture from the hot water, which passes with the air into the 
cylinders, assists in lubricating the valves and pistons.* 

Pipe-line and Charging Stations. The capacity of the com- 
pressed-air system naturally depends on the length of haul and 
size of locomotives, as influenced by the daily output, weight of 
trains, and gradients of the haulage lines. For short hauls, the 
pipe-line is sometimes omitted altogether, the locomotive return- 
ing each time to the compressor receiver to be recharged. In 
general practice, however, a pipe-line is carried underground, and 
at one or more points charging stations are established. The lo- 
cation and distance apart of these stations is determined by the 
haulage distances and the storage capacity of the locomotive 

* E. P. Lord, Paper Read before the Anthracite Coal Operators' Association, 
N. Y., Oct. 13th, 1897. 



COMPRESSED AIR HAULAGE FOR MINES 295 

tanks. It is evident that the last or innermost charging station, 
farthest from the compressor, must be at a point from which the 
locomotive can reach the end of its trip and return for a fresh 
supply of compressed air. For very long hauls, heavy traffic, 
or adverse gradients, a charging station may be required at each 
end of the line. 

It is unnecessary to provide receivers inside the mine, though 
this may be done advantageously if the diameter of the supply 
pipe is small. The pipe-line itself is intended to act as a storage 
reservoir, and should be of a diameter which, in proportion to its 
length, will furnish a cubic capacity sufficient to charge the 
locomotive tanks quickly and without serious drop in pressure. 
In other words, when the locomotive is connected with the pipe- 
line, and the charging valve opened, the pressure in the locomotive 
tank and in the pipe, on equalizing (as it must), should not fall 
much below the stated pressure which the locomotive is designed 
to carry. It is, therefore, desirable that the volume of storage, 
represented by the main — or main and receiver — should be at 
least three times the tank capacity of the locomotive. To de- 
termine the necessary storage capacity of pipe-line, or combined 
receiver and pipe-line, several variables must be harmonized, as 
follows:* 

V = storage volume required, in cu. ft. 
v = volume of locomotive tanks, in cu. ft. 
P = pipe-line pressure, in lbs. per sq. in. 
p = desired pressure in locomotive tanks, in lbs. per sq. in. 
ft = residual pressure in locomotive tanks, just before charging, 
in lbs. per sq. in. 

Then: V (¥-p) =v (p-p') f or V= V ^~ P>) 

For example, let P=9oo lbs., p = 7S° l DS -> P' = 12 5 l° s -> an d 

^ = 100 cu. ft., from which: 

__ 100 (750—125) , , . 

V = ii^ ^ =416.6 cu. ft. 

900-750 

By transposition, the same formula may be used for finding 

* H. K. Porter Co., " Handbook of Compressed-Air Haulage," 1907. 



296 COMPRESSED AIR PLANT FOR MINES 

the pipe-line pressure required to produce a given pressure in 
the locomotive tanks. When several locomotives are served by 
the same pipe-line and compressor it is rarely, if ever, necessary 
to design the system for charging more than one at a time. If 
the volumetric capacity of the pipe- line be ample, the relatively 
small drop in gauge pressure on charging is soon recovered by 
the compressor, which, except in plants operating a single locomo- 
tive, is kept in nearly constant operation. In case additional 
locomotives are required after the original installation of the 
system, the same pipe-line may still serve, provided the com- 
pressor be of sufficient size to charge it to full pressure at shorter 
intervals. 

The piping, which generally varies in diameter between 3 
and 5 ins. — sometimes 6 ins. — should be of the best material, lap- 
welded, and with sleeve joints made with the utmost care to pre- 
vent leakage. To stop leaks, the sleeves should have annular 
grooves at each end into which soft metal calking is driven if 
required. It is advisable not to bury the pipe alongside the 
track, but to carry it entirely uncovered along one side of the tun- 
nel or gangway, either on the floor or on brackets, so that leaks 
will at once attract attention and be stopped. While an occa- 
sional bend in the pipe-line is advantageous in permitting free 
expansion and contraction, they should not be too numerous, as 
they involve more joints and therefore a greater possibility of 
leakage. 

Charging Apparatus. A common form of apparatus for charg- 
ing the locomotives, as shown in Fig. 106, consists of a vertical 
right-angled connection inserted in the air main by means of a 
heavy tee. This connection has an arm projecting from the main 
a sufficient distance for conveniently coupling to the charging 
pipe of the locomotive. It comprises two parts: a vertical, 
rigid branch, containing a strong, accurately fitted i^-inch gate- 
valve, and a short horizontal pipe, attached to the valve by a 
union and a ball-and-socket or flexible joint, for coupling to the 
locomotive charging pipe. Thus, the locomotive need not be 
stopped at a precise point for charging, but has a foot or two lee- 



COMPRESSED AIR HAULAGE FOR MINES 



297 



way on its track. When not in use, the flexible connection is 
swung back, out of the way. In the locomotive connection there 
are usually two ball-and-socket joints, together with a check- valve 
close to the tank. 

After coupling on the locomotive, the gate-valve is opened, 
whereupon the air pressure immediately forces together the parts 
of the ball-and-socket joints and makes a perfectly tight COnneC- 




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298 COMPRESSED AIR PLANT FOR MINES 

tion. As soon as equilibrium is established between the pressures 
in the main and the locomotive tank the gate-valve is closed. To 
break the coupling, the compressed air remaining in the connect- 
ing pipe, between the gate-valve and the locomotive check-valve, 
must first be released. This is done by opening a small " bleeder 
valve," placed just above the gate- valve, as shown in the cut. 
The joints then become loose and are readily manipulated. The 
actual time occupied in charging is very short (usually about 
three-quarters of a minute), owing to the high pressure in the 
main and the relatively large diameter (ij in.) of the charging 
pipe; but, including stopping the locomotive and making the 
connection, 1 \ to 2 J mins. may be allowed. Frequently, charging 
may be done during the necessary delays in shifting cars and 
making up trains. 

Calculation of Motive Power. To determine the motive 
power required for a given output, several factors must be known, 
viz: the tractive resistance per ton of the loaded cars on a level, 
the resistances due to gradients and curves, the weight of empty 
and of loaded cars, and the number of cars to be hauled in each 
train. The values of these factors are known approximately or 
are readily ascertained, with the exception of the resistances due 
to curvature of track and character of roadbed. The former has 
been determined experimentally for ordinary surface railways, 
but underground mine track is apt to be roughly laid, with curves 
of varying and irregular radius, and the elevation of the outer rail 
improperly adjusted. With sufficient weight on the drivers, 
however, sticking on a curve may be avoided, in the case of com- 
pressed-air haulage, by temporarily admitting to the cylinders a 
little air at full tank pressure, as already noted. In this respect 
compressed-air locomotives possess a material advantage over 
those driven by steam, in which the working pressure is limited 
and practically constant. 

The average tractive force required per ton depends not only 
on the physical condition of the track and roadbed, but on the 
character and state of repair of the running gear of the cars. On 
level mine track the coefficient of rolling friction should usually 



COMPRESSED AIR HAULAGE FOR MINES 299 

be taken at from thirty to forty pounds per ton, though it may 
be considerably higher on poorly laid or light track, or at the in- 
stant of starting the load. With mine track in exceptionally good 
condition, the coefficient may be as low as twenty pounds per ton. 
The grade resistance is twenty pounds per short ton, for each one 
per cent, of grade. Not infrequently, the distribution of grades on 
the haulage lines is such that the maximum load is not the resist- 
ance of the loaded trains, which are usually hauled on slight down 
grades, but that of the return trains of empty cars on the adverse 
gradients. To obtain the most economical results, gradients 
should be not over J to f of 1 per cent, in favor of the loaded trains. 
With mine track and rolling stock of ordinary character, and a 
grade of 5 to 6 ins. per 100 ft., the coefficient of rolling friction is 
nearly the same for a loaded train hauled down as for an empty 
train of the same number of cars hauled up the grade. Heavier 
and even adverse grades often become necessary — sometimes as 
steep as 2 J per cent, to 3 per cent, or more, but they should be 
avoided as far as possible, because the maximum tractive force of 
the locomotive falls off rapidly. On a 2 J-per-cent. adverse grade 
the locomotive can haul only about 4 times its own weight, even 
if the track be not slippery. Grades should be reduced on curves. 
Colliery cars, carrying 2 J to 3 \ tons, will weigh from 1,800 to 
2,300 lbs., while those used in metalliferous mines, where me- 
chanical haulage is employed, vary between, say, 1,000 and 2,000 
lbs. Many cars of the last-named weight are in use, for example, 
in the iron mines of the Northwest. Finally, having ascertained 
as near as possible the values of the different factors, the proper 
allowance of reserve power, in terms of volume and pressure of 
air, to cover indeterminate additional resistances due to imper- 
fections of track and rolling stock, is a matter of judgment and 
experience. 

With a given air pressure, the capacity required for the loco- 
motive storage tanks depends primarily on the length of round 
trip to be made with a single charge of air. W T hen this distance is, 
say, 1 to 1 J miles, the tank capacity generally varies between 50 
and 150 cu. ft., according to the load; which, in turn, together 



300 COMPRESSED AIR PLANT FOR MINES 

with the track and grade resistances, governs the dimensions of 
the cylinders. Cylinders of 5 ins. X 10 ins. up to 9 ins. X 14 ins. 
are commonly used for mine service, the larger sizes being 
adopted for heavy work in collieries. Still more powerful locomo- 
tives are used for some kinds of surface work. In several installa- 
tions, as at mines of the Philadelphia & Reading Coal & Iron Co., 
the compressed-air locomotives have been designed with com- 
pound cylinders. For long runs, of over one and one-half miles, it 
is often desirable to increase the air pressure, rather than build 
tanks of very large size. Another plan is to provide a tender, 
which carries one or more auxiliary tanks, connected with those 
on the locomotive. Very long runs can be made by this means. 

Having determined the total work in foot-pounds to be done 
with a single charge of air, on a run of the maximum length, 
specifications may be obtained from the builders for a loco- 
motive of suitable weight, gauge, wheel-base, tank capacity, and 
cylinder dimensions. 

Compressors for Charging Pneumatic Locomotives. For 
compressing the air to the high tension required by pneumatic 
locomotives, the work must be done in at least 3 stages; 
4-stage compressors are sometimes employed for pressures ex- 
ceeding 900 or 1,000 lbs. Efficient intercoolers are of course 
placed between the successive cylinders and an aftercooler is 
desirable. Fig. 107 shows the standard type of 3-stage locomo- 
tive charger built by the Norwalk Iron Works Co., for pressures 
up to 1,000 or 1,200 lbs. The air passes from the low-pressure 
cylinder to the lower of the two intercoolers and, thence to the 
intermediate cylinder. From the latter the air is delivered 
through the vertical pipe to the upper intercooler, whence it 
passes through the inclined pipe to the high-pressure cylinder. 
From this cylinder the compressed air is delivered to the receiver 
through the connection indicated under the outer end of the 
cylinder. Other compressors by the same builders are designed 
for pressures up to 3,000 and 4,000 lbs. 

The air end of a three-stage, tandem locomotive charger, 
built by the Ingersoll-Rand Co., is shown in longitudinal section 



COMPRESSED AIR HAULAGE FOR MINES 



301 



in Fig. ic8. The high-pressure intercooler is placed in the lower 
right-hand corner of the cut. Figs. 109 and no illustrate respec- 
tively the low- and high-pressure air ends of a duplex, four-stage 
compressor. In Fig. 109 are the intake and first intermediate 
cylinders, and in Fig. no the second intermediate and high-press- 
ure cylinders. A perspective view of a large compressor of this 
type is shown by Fig. in. 

It will be seen in the sections that the pistons of the high-press- 
ure cylinders are solid rams or plungers, provided with a series 




Fig. 107. — Norwalk Locomotive- Charging Compressor. 



of packing rings. These, with the high-pressure valves, must be 
made with special care, to prevent the serious effects of leakage 
of high-pressure air. Even a small percentage of leakage will 
greatly reduce the volumetric capacity and efficiency. Loco- 
motive chargers are also built by the Sullivan Machinery Co. 
and others. 

When the mine is already provided with an ordinary low- 
pressure air plant, for machine drills and other service, and which 
has some surplus capacity, a two-stage charging compressor may 
be installed, to take air from the low-pressure system and bring it 



3 02 



COMPRESSED AIR PLANT FOR MINES 




bo 

u 



o 

i-J 

<u 
be 
03 

-4-> 

en 



H 



bo 

c 



00 

O 



o 



COMPRESSED AIR HAULAGE FOR MINES 



;°3 



r \ 




p^M^yr^^-'^ 



f; fe^^3S^E3 




Figs. 109 and no. — Ingersoll-Rand Four-Stage Locomotive Charger. 



3°4 



COMPRESSED AIR PLANT FOR MINES 



up to the tension required for the locomotives. By this arrange- 
ment some reduction in the cost of the plant may be effected. 
Care must be exercised, however, in making such a combination. 
If the quantity of air produced by the low-pressure system should 
at times be insufficient to furnish the necessary excess, at ordinary 
gauge pressure, for the locomotive-charging compressor, the latter 
might be compelled to compress from too low an initial pressure. 



'jjftt ^k 


Mffllv 






H JUr 1 '■ m H 


jA ^ .^ #^^ 


MB 






■*LL?$*fm\ Wi&k U JBF* "'^% --£* 


^^^"C.. ^* m^i . **5 


■'I 






■■■ / m& t ' '"^'"-"■-'^■^■.,.'"'" .~*x?"*'--- 




.«*»PBBi^ 


:'-;>r-V 




"i $'■ *■■ r- 


~^> \ 1 




9ggD^^^ 


-; lflfl 1 















Fig. hi. 



This would cause excessive development of heat and, aside from 
the difficulty of maintaining proper lubrication, might possibly 
raise the cylinder temperature to the flashing-point of the 
oil, thus causing an explosion. This matter has been discussed 
in Chapter XIV. Generally, it is preferable to install an inde- 
pendent locomotive charger. With such a compressor, the final 
temperature can be kept down to a moderate degree, say, 200 
to 300 F., provided the plant is not too small for its work. The 
compressor should be run at a moderate speed, and as the de- 
mand upon it is usually somewhat irregular, causing frequent re- 
ductions of speed, and even occasional stoppages, the air cylinders 
are prevented from becoming over-heated. 



COMPRESSED AIR HAULAGE FOR MINES 305 

The capacity of the charging compressor depends on the pipe- 
line pressure to be maintained, the number of locomotives to be 
operated, the cubic contents of the locomotive tanks, the pressure 
carried by the system, and the relation between the charging 
periods. 

Let C= compressor capacity required, in cubic feet of free air 
per minute. 
L= locomotive-tank capacity, in cubic feet of free air per 

minute. 
N = number of charges required in any given time, T. 

• r* NL 

Hence the equation: U=-=- 

For example, if N = 3, L = 5,200 (corresponding to 100 cu. 

ft. of air at 750 lbs. gauge pressure), and T = 60 minutes : 

„ ^Xs,20o , . . 

L = - — y =260 cu. ft. free air per minute. 

60 

When the locomotives are charged — as they usually can be — 
at approximately equal intervals of time throughout the day, a 
single application of the above formula will be sufficient. Other- 
wise, calculations are required to determine the maximum and 
minimum rates of consumption of air. It is hardly necessary to 
add that, when the plant is installed at an altitude above sea- 
level, allowances must be made for decreased output, as ex- 
plained in Chapter XIII. 

Examples of Compressed-Air Haulage Plants. In further 
illustration of this subject, some of the details of a few successful 
installations may here be given. 

1. At the Buck Mountain Colliery, Penn., are two 8-ton H. 
K. Porter locomotives, each with 2 tanks, respectively, 1 5 and 1 7 
ft. long, having a combined capacity of 130 cu. ft. of air at 550 
lbs. pressure.* The cylinders are 7 ins. X 14 ins.; wheel-base, 5 
ft. 3 ins. ; gauge of track, 42 ins. ; height, 5 ft. 2 ins. ; length over 
all, 19 ft. A round trip of 5,100 ft. is made in 30 to 40 minutes, 
or 2,500 ft. in 12 to 15 minutes, with trains of 12 cars, on grades of 
\ to 4J per cent., averaging j of 1 per cent, in favor of the load. 

* Mines and Minerals, July, 1898, p. 538. 
20 



306 COMPRESSED AIR PLANT FOR MIXES 

One locomotive delivers 150 cars per 10 hours, doing the work 
formerly done by 15 mules. Weight of cars, 3,400 lbs. empty, and 
10,400 lbs. loaded. A 3-stage Xorwalk compressor supplies 
375 cu. ft. free air per minute, at 700 lbs. gauge. Pipe-line, 4 ins. 
diameter and 9,600 ft. long, with a storage capacity of 800 cu. ft. 

Average cost per ton-mile: 1.875 cents for the gross weight 
hauled, or 3.77 cents for net weight of coal. The cost for mule 
haulage under the same conditions was formerly 3.94 and 7.92 
cents, respectively. 

The cost of this plant was as follows : 

Two locomotives. $5o°5 * 

Air line: 9,647 ft. 4 in. pipe 82,894. 

Six charging stations 360 . 

Fittings and valves 382 . 

Labor cost for erection 998. 

4,634. 

Compressor 82,880 . 

Sundries and erection 246 . 

Compressor house 256. 

Steam line to compressor 152. 3,534- 



Total cost 813,673. 

2. Empire Mine, Grass Valley, Cal. Several small com- 
pressed-air locomotives, built by Edward A. Rix, are employed in 
the deep levels of the mine, for hauling trains of 5 cars, each carry- 
ing 1 ton. The maximum distance covered by a round trip is 
about 5,000 ft. Locomotive storage tank measures 36 ins. 
diameter X 48 ins. long, carrying a pressure of 500 lbs. The di- 
mensions over all are only 5 ft. long X 30 ins. wide X 52 ins. high, 
the gauge of track being 18 ins. One of these locomotives (Fig. 
112) is operated by a pair of vertical engines, a chain and sprocket 
drive connecting the crank-shaft with the rear axle. There are 
2 tandem tanks, one of them being carried on a tender. A re- 
heater, provided with a Primus kerosene burner, reheats the air 
after its pressure has been reduced in the auxiliary reservoir. 
Mr. Rix has recently built 3 similar locomotives, but with a single, 
larger tank, for a 3-mile tunnel, near San Francisco. They carry 




o 

I— I 



308 COMPRESSED AIR PLANT FOR MINES 

1,000 lbs. tank pressure, the working pressure being ioolbs. ; 
each locomotive making a 2 -mile round trip, at 6 to 7 miles per 
hour.* 

3. The Peerless Colliery, Vivian, West Va., operated for 
years several H. K. Porter locomotives, with 5Xio-in. cylinders 
and weighing 10,000 lbs. Over all dimensions: 10 ft. 5 \ in. long 
X 5 ft. 8 ins. wide X 4 ft. 5 ins. high. Four driving wheels, 23 ins. 
diameter ; gauge, 44 ins. Capacity of main storage tank, 47 cu. ft. ; 
pressure, 535 lbs.; charging time, 20 seconds; working pressure, 
125 lbs. Pipe-line, 3 ins. diameter, with a total capacity of 242 
cu. ft. Line pressure, 735 lbs. Trains consist of 6 cars, each 
weighing loaded 8,500 lbs. Grades range from level to 2 \ per 
cent., generally in favor of the load. Curves from rooms to haul- 
ageways, 23 ft. radius, though locomotives are designed to work on 
curves as sharp as 15-ft. radius. Length of maximum round trip, 
9,000 ft. ; maximum speed 10 to 12 miles per hour. Cost of each 
locomotive, $1,800. 

4. The following data, concerning one of the plants of the 
Philadelphia & Reading Coal & Iron Co., and compiled by Mr. 
G. Clemens, a division engineer of the Company, are published 
in the catalogue of the Baldwin Locomotive Works : 

a. Shaft level — 1 locomotive. 

Round trip, 5,200ft. ; grades x\ to x 8 o" °f * P er ccn t., all in favor 
of load; charging station at each end of run; gauge of track, 44 
ins.; 40-lb. rails; weight of cars — empty, 3,300 lbs., loaded, 8,800 
lbs.; from 15 to 38 cars per trip; total output, 600 cars per 10 
hours. Round-trip time, 12 min.; charging time, 1 min. A 
round trip and a half can be made with one charging. 

b. Slope level — 1 locomotive. 

Length of haul, 3,200 ft., of which 700 ft. is on a slope whose 
grade ranges from 4 T 1 - g - to 5 -J per cent. Grade of main gangway, 
t 4 q- to t 8 -q of 1 per cent., in favor of load. Trains of 10 cars are 
hauled on main gangway, and 4 cars on the slope; weights of cars 
same as above. 

Locomotive-tank pressure at start, 600 lbs. ; at end of trip, 

* Compressed Air Magazine, Feb., 1908, p. 4747. 



COMPRESSED AIR HAULAGE FOR MINES 309 

200 lbs. Average working pressure, 180 lbs. The cost of the 
plant was as follows : 

One Norwalk 3-stage compressor, erected $5, 180. 74 

Pipe-line, 4,200 ft., 5 in., including 3 charging stations 2,951 .06 

Two Baldwin compressed-air locomotives and fittings 4,904.33 

Alterations in gangways to adapt them for locomotive haulage 665 . 1 7 

Total cost $13,701 . 30 

Daily operating cost, for 180 days in the year $14 . 69 

Fixed charges, depreciation, repairs, etc., figured at 10 per cent., together 

with cost of steam power 9 . 00 

Total running expenses per day $23 . 69 

Cost per car, at 660 cars per day 3.6 cents 

Previous cost of mule haulage per car 5.1 " 

Saving per year, about $1,800.00 

5. At the Wilson Colliery, of the D. & H. Coal Co., a large 
locomotive was installed by the Dickson Manufacturing Co., hav- 
ing six 26-in. drivers; wheel-base, 7 ft. ; cylinders, 9 ins. X 14 ins. ; 
gauge of track, 30 ins. The locomotive carries two tanks, 18 ft. 
6 ins. and 15 ft. 6 ins. X 30 ms. diameter, with a capacity of 160 cu. 
ft. of air at 600 lbs. Pipe-line, 4,100 ft. long; pressure, 700 lbs. 
Total charging time, 1 min. 25 sees. After reduction to 125 lbs. 
working pressure the air is reheated. Trains usually consist of 
30 cars, each weighing loaded, 5,850 lbs., though the locomotive 
has a capacity of 50 cars. Grades, from 9 ins. per 100 ft. against 
the load, to 12 ins. per 100 ft. in favor of the load. Round-trip 
time, for 8,200 ft. plus a switching distance of 800 ft., 16 min. 
Cost of haulage per ton-mile, gross, about 1 J cents. 

6. The Anaconda Copper Mine, Butte, Mont., is provided 
with a number of compressed-air locomotives with 5-in.Xio-in. 
cylinders and weighing 10,000 lbs. Over all dimensions: 
height, 58 ins. ; width, 58 ins. ; length, 10 ft. 4 J ins. ; four driving 
wheels, 23 ins. diam. ; wheel-base, 36 ins., designed for curves of 
12-ft. radius. Capacity of main tank, 47 cu. ft. ; pressure, 550 lbs. 
working pressure, 125 lbs.; charging time, 60 sees. Length of 
haul, 2,400 ft. round trip; load, 6 cars, weighing loaded 3,45° ^ s * 



310 COMPRESSED AIR PLANT FOR MINES 

each; track nearly level. The locomotives are designed to make 
2 round trips, or 4,800 ft. on 1 charge, with cold air; but, by re- 
heating with hot water, 3 round trips can be made. 

At the new reduction works of the Anaconda Company, there 
are 13 H. K. Porter locomotives, employed in handling the prod- 
ucts between the different divisions of the plant, which covers 
roughly an area of 2,200X2,300 ft., the length of haul ranging from 
1,000 to 7,000 ft. Twelve of the locomotives have the following 
dimensions; weight, 26,000 lbs.; cylinders, 9 \ X14 ins.; driving 
wheels, 28 ins.; wheel-base, 54 ins.; main tanks, 132 cu. ft.; 
draw-bar capacity, 5,700 lbs. Another locomotive weighs 
42,000 lbs.; cylinders 12X18 ins.; driving wheels, 36 ins.; wheel- 
base, 60 ins.; main tanks, 218 cu. ft.; draw-bar pull, 9,180 lbs. 
Tank pressure, 700 to 800 lbs.; working pressure, 150 lbs.* 

7. The Homestake Mining Co., Lead, S. D., employ under- 
ground 10 H. K. Porter locomotives, weighing 9,500 lbs. and 
measuring over all, 4 ft. 11 ins. high X3 ft. 3 \ ins. wide X 10 ft. 6 
ins. long. Gauge of track, 18 ins. They have a detachable rear end 
(similar to those of the Loretto Iron Co., mentioned in the 5th 
column of Table XXXV) to permit of transferring them from 
level to level, on a cage with a 9-ft. platform. At the same mine 
a small locomotive, with 5X8-in. cylinders (see Table XXXV) 
has been recently installed. This size is found more satisfac- 
tory, for the underground conditions prevailing in the mine, than 
the larger locomotive, with 6Xio-in. cylinders. 

8. Several 4-cylinder, Vauclain compound air locomotives, 
built by the Baldwin Locomotive Works, are in use in one of the 
collieries of the P. & R. C. & I. Co.f Cylinders 5 and 8 ins. X 12 
ins. stroke, with valves of the balanced-piston type. Tank press- 
ure, 600 lbs. ; working pressure, 200 lbs. Driving wheels, 24 
ins. ; wheel-base, 48 ins. ; storage tanks, of j-q in. plate, 1 1 ft. 4 J ins. 
and 13 ft. 7 \ ins. X 31 ins. diameter; auxiliary reservoir, 8 ins. 
diam. X 7 ft. 4 ins. long. Over all dimensions: 6 ft. 4 ins. wideX 

* A detailed description of this haulage plant is given by C. B. Hodges, Cassier's 
Magazine, 1905. 

t Engineering and Mining Journal, Aug. 19th, 1899, p. 218. 



COMPRESSED AIR HAULAGE FOR MINES 3II 

14 ft. long X 6 ft. 6 ins. high; weight, 22,000 lbs. Trains of 32 
cars, each weighing loaded about 9,000 lbs., are hauled on if- 
per cent, grade, in favor of the load. 

9. At the Aragon Iron Mine, Norway, Mich., is an H. K. 
Porter locomotive. Weight, 7 tons ; height, 5 ft. 2 ins. ; width, 

4 ft. 2 ins.; length, 12 ft., over all. Four 24-in. drivers; wheel- 
base, 48 ins. ; gauge, 22 J ins. ; cylinders, 7X12 ins. ; tank pressure, 
700 lbs. ; working pressure, 140 lbs. ; charging time, 30 to 60 sees. 
Haulage distance, from 1,200 to 4,000 ft. ; pipe-line, 1,800 ft. ; in- 
cluding 750 ft. down the shaft. Locomotive hauls four 20-car 
trains per 10 hrs., from each of 10 loading places. Weight of 
loaded train, including locomotive, 43 tons; weight empty train, 18 
tons. Compressed air is furnished by a Norwalk 3 -stage charger : 
steam cylinders, 14X16 ins.; air cylinders, 10J, 7 J, and 2f ins. X 
16 ins., compressing 125 cu. ft. free air per minute to 800 lbs. 
At the compressor there are two receiver storage tanks, each 
3 x17 ft. 

10. Compressed-air haulage plant at No. 6 Colliery of the 
Susquehanna Coal Co., at Glen Lyon, Penn. Following is an 
abstract of tests made by J. H. Bowden and R. V. Norris.* 
Though the plant is not of the latest pattern, the results given will 
be found useful. 

Compressor: Norwalk, 3 -stage; steam cylinder, 20X24 ins. ; 
air cylinders, 12 J, 9J, and 5 ins. X 24 ins.; capacity, at 100 revo- 
lutions, 296 cu. ft. free air per minute, compressed to 600 lbs. 
Main pipe-line at No. 6 shaft, 4,380 ft. long, 5 ins. diameter, with 

5 charging stations, and capacity of 608 cu. ft. Branch line, in 
No. 6 slope, 3,100 ft. long, 3 ins. diam., with 3 charging stations, 
and capacity of 159 cu. ft. 

Locomotives: two, by H. K. Porter Co. ; weight, 8 tons; tank 
capacity, 130 cu. ft.; pressure, 550 lbs. reduced to 160 lbs. in an 
8-in. auxiliary reservoir, of 4.2 cu. ft. capacity. Cylinders, 7X14 
ins. ; four 24-in. drivers. 

At No. 6 shaft the run averages 4,000 ft. each way, on J to 2f- 
per cent, grades, averaging about 1 per cent, in favor of the load. 

* Transactions American Institute of Mining Engineers, Vol. XXX, p. 566. 



312 



COMPRESSED AIR PLANT FOR MINES 



Run at No. 6 slope averages 2,100 ft., with nearly the same 
grades. Cars weigh 2,800 lbs. empty, and about 9,800 lbs. 
loaded, and are hauled in trains of 12 to 20. The shaft loco- 
motive hauls about 355, and the slope locomotive 320 cars, per 10 
hours, doing the work of 32 mules. Tests on the compressor 
showed 150 indicated horse-power at 131 revolutions, compress- 
ing 387.8 cu. ft. free air per minute. 

The combined capacity of both pipe-lines is 767 cu. ft., 
which, at 600 lbs. gauge pressure, is equivalent to 32,500 cu. ft. 
free air. Capacity of locomotive main and auxiliary tanks, 134.6 
cu. ft. At 508 lbs. (at which pressure the tanks equalize with the 
mains, the initial pressure being 600 lbs.), this is equivalent to 
4,845 cu. ft. free air. In standing 12 hours, the pipe-line pressure 
falls to 350 lbs., the loss per hour from leakage thus being 974 cu. 
ft. free air, or 4.18 per cent, of total air compressed. 

Table XXXVII 

Air Consumption 



Shaft Loco. 






Slope 


No. 2 
Plane. 


No. 3 
Plane. 


Loco. 


3 


10 


16 


3 


10 


15 


15-33 


12.7 


11. 4 


13 

1,724 

1,631 
3,355 


13 
5,686 
1,898 
7,584 


XI -3 
1,230 

599 
1,829 


113 


7i 


2C 


3 


128 



Number of trips, empty 

Number of trips, loaded , 

Average number cars per trip, empty 

Average number cars per trip, loaded 

Average cu. ft. free air per trip, empty 

Average cu. ft. free air per trip, loaded 

Average cu. ft. free air per round trip 

Average cu. ft. free air per ton-mile, on gross tonnage 
Average cu. ft. free air per ton-mile, on net tonnage . 



Average volume free air used by both locomotives per ton- 
mile was : gross, 100 cu. ft. ; net, 180 cu. ft. The greater quantity 
of air used by the shaft locomotives is due to the heavier grades and 
switching required. Another test showed a total consumption of 
223,020 cu. ft. free air, for hauling 676 cars per day. The volume 
of free air apparently compressed for this work was 279,200 cu. 



COMPRESSED AIR HAULAGE FOR MINES 



3^ 



ft., of which 83.4 per cent, is accounted for, leaving 16.6 per 
cent, for leakage and slip in the compressor, leakage in air lines, 
and changes in temperature. 

The cost of the plant, omitting boilers, was : 

Compressor and extras $2,955 - 75 

Two locomotives and extras 5,869 . 76 

Pipe-line: 5-in. line, 6,000 ft $2,914.32 

3-in. line, 4,000 ft 1,240.46 4,154.78 

Steam connections to compressor 278.27 

Material and labor, compressor house and foundations, installing pipe- 
line, etc 1,525-23 

Charging stations 372 .21 

Total cost -^ $15,156.00 

The average cost of operation of entire plant, for 2 years, on 
basis of 170 days' work per year, was $12.60 per 10-hour shift, 
including an allowance of $2.32 for steam for compressor, fur- 
nished by main boiler plant of mine. Adding proportion of 
fixed charges, with interest, depreciation and repairs, the daily 
cost, on basis of 300 days' work per year, would be $18.52 per 
day. For the 2 years, the average cost per ton-mile was as follows : 

Table XXXVIII 

Operating Costs 





1897 (179 Days). 


1898 (160 Days). 


- 


. 

Q 



U 
>, 

Q 


O en 

So 


c 
. 

Q 


■*-» 

O 

u 

Q 


5 t/5 
0.1D 


Slope locomotive, gross tonnage 

Slore locomotive, net tonnage 

Both locomotives, gross tonnage 

Both locomotives, net tonnage 


1,485 
825 
648 
360 

2,i33 
1,185 


$11.12 
11. 12 
11. 12 
11. 12 
22.23 
22.23 


0-75 
i-35 
1.72 

3-°9 
1.05 
1.89 


1,521 

845 
720 

400 

2,241 

1,245 


$12.00 
12.00 
12.00 
12.00 
24.01 
24.01 


a. 79 
1.42 
1.67 
3.00 
1.07 
i-93 



In these two years the saving over the expense of the mule 



314 COMPRESSED AIR PLANT FOR MINES 

haulage, previously employed, was $14,218.00, or nearly the total 
cost of the haulage plant. 

11. Following is the cost of a large colliery plant, as given by 
Beverly S. Randolph,* who installed and afterward operated it: 

Three-stage, compound compressor $5,300. 

Pipe line: 5,600 ft., 5 in $5,600. 

3,100 ft, ?\ in 1,700. 

1,000 ft., 1^ in 300 . 

7,600. 

Two main locomotives, weight 30,000 lbs 6,000 . 

Five gathering-locomotives, weight 8,000 lbs 10,000. 

Two boilers, each 80-horse-power 1,000. 

Installation, labor, and material 4,000. 

Total cost $33,900 . 

This plant includes an unusually large number of small 
gathering-locomotives, for collecting cars from the individual 
workings and making them up into trains for the main haulage 
lines. If the locomotive equipment had consisted of four2 5,ooo-lb. 
engines, costing, say, $2,800 each, and which would do the same 
work, the total cost of the plant would be reduced to $29,100. 
This cost compares very favorably with that of electric-haulage 
plants of the same capacity. 

* Transactions Institution of Mining Engineers (England), Vol. XXVII (1904), 
P- 433- 



INDEX 



Abrams, H. T., test on air-lift pumps, 277 

Absolute pressure, temperature and zero, 43 

Adiabatic compression, 44, 48-51, 137-139, 142, 143; equation of, 50, 61, 89 

Adiabatic expansion, 213 

Adjustable steam cut-off valve, 30 

Aftercooler, 146, 149 

Ainsworth (B. C), hydraulic air compressor at, 189, 190 

Air and steam cards combined, 31 

Air card, 49, 98; elements of, 141, 143; of wet and dry compressors, 51; of stage 
compressor, 88 

Air-cataract valves, 113 

Air compression: at altitudes above sea-level, 164-169; by direct action of falling 
water, 183-193 

Air compressors: belt-driven, 12, 32, 40; builders, list of, 42; chain-driven, 12, 
40; classification of, 8, 9, 10, 12; dry, 50, 51, 58-69; for compressed-air haul- 
age, 300-305; geared, 40, 41; half-duplex, 16; horse-power of, 134, 135; 
hydraulic, 183-193; makers of (see Compressors); performance of, 133-143; 
water-driven, 32, 34-40; wet, 50-52, 57 

Air engines, 207—219, 228, 230, 236 

Air governors, 150-163 

Air hammer drills, 249 

Air inlet, area of, 93, 102; conduit for, 108, 109 

Air inlet valves, 91-109 

Air-lift pumps, 265, 270-281 

Air pressure for machine drills, 246, 247 

Air pressure regulators, 150-163; American, 152; Clayton, 151, 152, 155; In- 
gersoll-Sergeant, 160, 161; Laidlaw-Dunn-Gordon, 161, 162; Norwalk, 153, 
154; Rand, 157; Sullivan, 158, 159 

Air receivers, 144-149; functions of, 145, 146; sizes, 144; underground receivers, 

147 

Allis-Chalmers compressors, 12, 42, 92 

Allis-Chalmers mechanically controlled valve-motions, 123, 124 

Altitudes above sea-level, compression at, 164-169 

American Air Compressor Works, 42 

American air-pressure regulator, 152 

American Institute of Mining Engineers, Transactions of, 177, 180, 311, 314 

American Locomotive Co. compressed-air locomotives, 288 

American Machinist, 88, 163, 165, 201, 218, 237 

Anaconda Copper Mine, compressed-air haulage at, 309, 310 

Angelo and Cason Mills, South Africa, tests on air-lift pumps, 277-279 

Anthracite Coal Operators Association, Transactions of, 294 

Aragon Iron Mine, Mich., compressed-air haulage at, 311 

Area of air inlet, 93, 102 

3*5 



316 INDEX 

Area of discharge valves, 114, 115 

Association of Engineering Societies, Transactions of, 262 

Auxiliary reservoirs for compressed-air locomotives, 292, 293, 294 

B 

Baffle plates for air receivers, 83, 149 

Bailey & Co., Manchester, piston valve, 132 

Baldwin Locomotive Works compressed-air locomotives, 286, 288, 290, 291, 308, 310 

Ball-and-socket joints for compressed-air locomotive charging-station, 296,297 

Behr, H. C, 219; air-lift pump experiments, 274 

Bell, J. E., experiments by, 245 

Belt -driven compressors, 12, 32, 40 

Bendigo district, Victoria, Lansell's air-lift pump, 279-281 

Bends in air pipe, 206 

Bjorling, P. R., 53, 66 

Bleeder valve for compressed-air locomotive charging-station, 297, 298 

Bowden, J. H., test on compressed-air haulage plant, 31 1-3 13 

Boyle's law, 44 

Buck Mountain Colliery, compressed-air haulage at, 305, 306 

Burleigh compressor, 2 

Burning-point of cylinder oils, 173, 175 

Burra-Burra Mine, compressor at, 19 

Butte, Montana, compressor explosion, 180 

By-pass for air cylinder, 66 



Cable-reel electric locomotive, 284 

Calumet and Hecla Copper Mine, 3 

Cam -controlled poppet inlet valve, 130, 131 

Canadian Electrical News, 189 

Canadian Engineer, The, 184 

Canadian Mining Institute, Transactions of, 223 

Capacity of air for moisture, 220 

Cards, air, 31, 47, 49, 51, 63, 88, 98, 141, 165, 209 

Carper, J. B., experiments by, 247 

Cason and Angelo Mills, South Africa, tests on air-lift pumps, 277-279 

Cassier's Magazine, 310 

Cataract valves, 112, 113 

Causes of freezing of moisture in compressed air, 221 

Cave rock drill, 1 

Chain-driven compressor, 12, 40 

Champion Iron Mine, experiments at, 246 

Channing, J. Parke, 19 

Charging compressor for compressed-air locomotives, 300-305 

Charging stations for compressed-air locomotives, 294-298 

Charles's law, 45 

Chattering of inlet valves, 94, 96 

Chodzko, A. E., 219 

Choking of air pipes by ice, 221, 223, 224 

Christensen compressor, 41, 42 

"Cincinnati" air-valve gear, 113, 122, 123 

Clack valves, 91, 105 

Clark, D. K., 44 

Classification of compressors, 8, 9, 10, 12 



INDEX 317 

Clayton compressor, 2, 42; governor for, 151, 152, 155 

Clearance: in compressor cylinder, 62-66, 89, 92, 169; in air engine, 212-217 

Clemens, G., 308 

Clifton Colliery, England, explosion in compressor, 174, 175, 181 

Coal cutters, compressed-air driven, 5 

Colladon, 1, 55 

Colliery Guardian, 223 

Comparison of types of compressors, 17, 19, 22 

Complete expansion, working with, 210, 213 

Compound compressed-air locomotives, 300-310 

Compound steam-end for compressors, 22, 24, 27, 29 

Compressed-air drills, 240-249 

Compressed-air engines, 207-219 

Compressed-air haulage, 282-314 

Compressed Air Machinery Co., 32, 42 

Compressed Air Magazine, 63, 115, 149, 177, 193, 198, 216, 218, 254, 268, 277, 308 

Compressed-air pumps, 223, 252-264, 265; adjustment of air pressure, 259; ef- 
ficiencies of, 258-260; preheating for, 261-263; prevention of freezing in, 
221, 223, 260 

Compressed air, reheating of, 225-239 

Compressed air versus electric transmission, 5, 6; versus steam transmission, 3, 4, 5 

Compressed air versus steam for direct-acting pumps, 252-254 

Compression curve, construction of, 140 

Compression of air, 43; at altitudes above sea-level, 164-169; by direct action 
of falling water, 183-193; heat of, 45; losses in, 44; stage compression, 71, 

137, 138 

Compressors, makers and names of: Allis-Chalmers, 12, 42, 92; Burleigh, 2; 
Chicago Pneumatic Tool Co., 42; Christensen, 41, 42; Clayton, 2, 42; De 
la Vergne, 30; Dubois-Francois, 2, 52, 91; Franklin, 42, 159; Hanarte, 54; 
Humboldt, 52, 53, 103, 113; Ingersoll-Rand, 9, 13, 30, 32, 37, 41, 42, 152, 300, 
302, 304; Johnson, 65, 102; King-Riedler, 10, 16; Laidlaw-Dunn-Gordon, 
9, 10, 11, 12, 14, 15, 42, 58, 59, 60, 92, 95, 113, 121-123; Leyner, 12, 20, 2^, 
42, 63, 106, 231; Nordberg, 19, 42, 58, 59, 92; Norwalk, 2, 12, 19, 42, 75, 76, 
92,99,300, 301, 306, 309, 311; Rand, 2, 3, 62, 91, 156; Rand and War- 
ing, 30; Riedler, 12, 24, 25, 113, 114; Sullivan, 12, 42, 81, 82, 124, 158, 233, 301 

Congelation of moisture in compressed air, 69, 220-224, 260, 261 

Consumption of air: by air engines, 216, 217, 228-230; by direct-acting pumps, 
255-258; by machine drills, 242-247; by pneumatic displacement pumps, 
267-268 

Conveyance of compressed air in pipes, 194-206 

Cooling: modes of, 48, 51; in receivers, 145, 146, 224 

Corliss air valves, 92, 116, 117, 123-125, 163 

Corliss steam-valve motion for compressors, 19, 26, 27, 39 

Couch, J. J., machine drill, 1 

Cox, Wm., 198, 254 

Cummings, Chas., system of compressed-air transmission, 218, 219, 264, 269 

Cushioning in machine drills, 246, 249 

Cut-off in air engines, 212-217; nominal and actual, 214-216 

Cylinder dimensions of simple pumps, 254, 255 

Cylinder proportions for compressors, 31, 32 

Cylinder volumes: in stage compression, 77-80; of air engine, 216, 217 



"Dancing" of inlet valves, 94, 96 

D'Arcy's formula for loss of pressure in pipes, 197-201 

De Kalb, 111., tests on air-lift pump, 274 



318 IXDEX 

Deliver}- valves, no-115; cataract-controlled poppets, 112, 113; effect of leakage, 

no; poppet type, no, 120, 122, 123 
Deposition of moisture from compressed air, 220—223 
Dickson Manfg. Co. compressed-air locomotives, 309 
Dingier Machine Works, Zweibruecken, 104 
Direct-acting pumps, operation by compressed air, 250-264 
Direct compression by falling water, 183-193 
Discharge valves (see Delivery valves) 
Discharge-valve area, 114, 115 

Displacement pumps, pneumatic, 265-270; consumption of air by, 267, 268 
Dover Iron Co. compressor, 132 
Drills, rock: air pressure for, 32, 245-247; reheaters for, 236; valve motion of, 

248, 249 
Drummond Colliery, compressed-air pumps at, 22^ 
Dry compressors, 50, 51, 58-69 
'"Dry" reheaters, 235, 239 
Dry versus wet compression, 66—70 
Dubois-Francois compressor, 2. 52, 91 
Duplex compressors, 9, 10, 12, 17, 19, 22, 23 
Duty of machine drills, 245, 247 



East Rand Proprietary Mines, Ltd.. tests on air-lift pumps, 277-279 

Ebervale, Luzerne Co., Pa., tunnel at, 203 

Efficiencies of air-lift pumps, 274—278 

Efficiencies of direct-acting compressed-air pumps, 258-260 

Efficiency of compressors, 19, 55, 84, 86, 98, 107, 134-139, 188, 192 

Efficiency of reheating, 227-230 

Electric versus compressed-air haulage for mines, 282-284, 314 

Electric versus steam locomotive haulage for mines, 282 

Empire Mine, Grass Valley, Cal., compressed-air haulage at, 306, 307 

Engineer, The (London), 277 

Engineering and Mining Journal, 184, 188, 192, 246, 263, 310 

Engineering yevi-s, 65, 274 

Expansion curves, air and steam, 208, 209 

Explosions in air compressors and receivers, 1 71-182 

Externally heated or "dry" reheaters, 235 



Fergie, Chas., 22^ 

Final temperature of air compression, 139, 140, 172, 173, 175, 176, 179 

Flash and ignition points of cylinder oils, 173. 175 

Four-stage compressor, 300, 301, 303 

Fowle machine drill, 1 

Franklin compressor, 42 

Franklin pressure regulator and unloader, 152, 159 

•' Free " air, 43 

Freezing of moisture in compressed air, 69, 220—224, 260, 261 

Frictional losses in compressors, 19, 73, 86, 133, 136, 214 

Frictional resistance in air pipes, 195-206; due to bends, 206 

Friedrich, G. C. H., tests on air-lift pumps, 276 

Frizell, J. P., 184 

Fuel cost of reheating, 229, 230, 231, 239 

Full pressure in air engines, working with, 2 10, 211 

Functions of air receiver, 145, 146, 148 



INDEX 319 



Gay-Lussac's law, 45 

Geared compressors, 40, 41 

Glen Lyon, Pa., Colliery, compressed-air haulage at, 311-313 

Goleta Mining Co. water-driven compressor, 32 

Governors, air, 150-163; American, 152; Clayton, 151, 152, 155; Ingersoll- 

Sergeant, 160, 161; Laidlaw-Dunn-Gordon, 161, 162; Norwalk, 153, 154; 

Rand, 157; Sullivan, 158, 159 
Grades of mine haulage tracks, 298, 299, 305-311 
Guttermuth air valve, 91, 104, 105 
Guttermuth, experiments on reheating, 230 
Gwin Mine, Cal., pump reheater, 263 



Halsey, F. A., 88, 165, 166, 201, 202 

Halsey pneumatic displacement pump, 270 

Hammer drills, air, 249 

Hanarte compressor, 54 

Harris pneumatic displacement pump, 269, 270 

Haulage by compressed-air locomotives, 282-314 

Heat curves, 47 

Heat losses in compressors, 67 

Heat of compression, 45, 46, 47-51, 56, 61, 71 

Heat, transference of, 48 

Heating of air-cylinder walls, 90 

Henderson, tests on air-lift pumps, 277-279 

Heron & Bury Manfg. Co., 42 

High-pressure transmission of air, as influencing freezing, 222 

"High-range" compressed-air transmission, 218, 219, 264, 269 

Hill, E., 177 

Hiscox, G. D., 214 

Hodges, C. B., 310 

Homestake Gold Mine, compressed-air locomotives at, 284 

Hoosac tunnel, 2, 204 

Horse-power: of air engines, 211-217; of air-lift pump, 275; of compressors, 86, 

iST-^o, i43> l88 > r 9 2 
Horse-power per cu. ft. of free air, 135, 138 
Humboldt compressor, 52, 53, 103, 113; rubber-ring valve, 103 
Humboldt Machine Works air-cataract valve, 113 
Humidity of atmospheric air, 220, 221 
Hydraulic air compressor, 183-193, 238 



Ignition-points of cylinder oils, 173, 175 

Ingersoll-Rand Co.: compressor, 9, 13, 30, 32, 37, 41, 42, 152, 300, 302, 304; 

tercooler, 81; receiver, 146; steam regulator, 155, 160, 161 
Ingersoll-Sergeant piston-inlet valve, 92, 101, 108 
Injection water: effects of, on air cylinder, 67; quantity of, 56; temperature of, 55, 

56 
Inlet air, arrangements for admitting, 108, 109 
Inlet valves, 90-109; area of, 93, 102; chattering of, 94 
Institution of Civil Engineers (London), Proceedings of, 184 



320 INDEX 

Intercoolers, 72, 75-90, 100; Ingersoll-Rand, 81, 86, 87; Leyner, 83, 85; Norwalk, 

76, 81; Schram, 84; Sullivan, 81, 82; volume of, 78, 84 
Internally fired reheaters, 234 
Isothermal compression, 45, 47, 48, 49, 50, 51, 133, 134, 135, 138, 139 



Jeddo (Pa.) mining tunnel, compressed-air transmission in, 203 
Johnson compressor, 65; air valves of, 102, 103 
Johnson, E. E., on performance of air-lift pump, 274, 275 
Joule's heat unit, 46, 142, 143 



Kennedy, Alex. B. W., reheating tests by, 229, 239 

King-Riedler compressor, 10, 16 

Knight water-wheel, 32 

Knowles Steam Pump Works, 42 

Kootenay (B. C), hydraulic air compressor at, 180, 190 

Koster piston air valve, 132 



Laidlaw-Dunn-Gordon Co. : compressor, 9, 10, 11, 12, 14, 15, 42, 58, 59, 60,92, 
95, 113, 121-123; "Cincinnati" valve gear, 113; mechanically controlled 
valve motions, 121, 122, 123; poppet inlet valve, 95; pressure regulators, 
161, 162 

Lansell's air-lift pump for shafts, 279-281 

Latta-Martin displacement pump, 268 

Leakage of compressed-air pipe lines, 198, 202, 205, 312, 313 

Leaky air piston, effect of, 89, 177, 179 

Leaky discharge valves, 176, 177, 179 

Lees, T. G., 61 

Lens Colliery, France, compressor air valve motion at, 130, 131 

Leyner compressor, 12, 20, 23, 42, 63, 106, 231; intercooler, 83, 85; reheater, 231 

Lightner Mine (Cal.), reheating at, 238 

Locomotives, compressed-air, 282-314; construction and operation, 284; cylinder 
pressures, 292, 294 

Locomotive charging compressors, 300-305 

Lord, E. P., 294 

Loss of head in pipe transmission, 195-204; of power, 194 

Loss of volumetric capacity due to piston clearance, 62, 63, 65, 66 

Losses in air compression, 133-136 

Losses in transmission piping, 194-206 

Lubrication of air cylinders, 69, 171, 173, 175, 177, 181 

Lubricators, sight-feed, 181 

M 

Machine drills, 240-249; consumption of air by, 242-247; duty of, 245-247; 

valve motion of, 248 
Magog (Province of Quebec) hydraulic air compressor, 184-188, 189 . 
Mallard, M., 210 
Mannesmann tubes for high air pressures, 291 



INDEX 321 

Mansfeld copper mines, underground receivers at, 147 

McKiernan Drill Co., 42; air-pressure regulator, 152 

McLeod, C. H., 188 

Mean pressures in air compression, 139, 143 

Mechanical Engineers' Assocn. of the Witwatersrand, Trans, of, 219, 247 

Mechanically controlled air valves, 91, 93, 116; Allis-Chalmers, 123, 124; American, 
126; Clayton, 126; disadvantages for delivery valves, 117, 123; for high alti- 
tudes, 168; Franklin, 126; Laidlaw-Dunn-Gordon, 122, 123; Nordberg, 118, 
120, 121; Norwalk, 118, 119; Riedler, 126-130; Rix, 126; Sullivan, 124, 125 

Menck and Hambrock cataract valve, 113 

Merrill pneumatic pump, 266-268 

Meyer steam valve gear, 9 

Meyer, R., air-cataract valve, 113 

Mines and Minerals, 192, 247, 305 

Mining and Metallurgy, 245 

Modern Machinery, 219 

Moist air, effect of, in compression, 67, 68, 69 

Moisture in air, 61, 69, 145, 147, 149, 220-224 

Mont Cenis tunnel, 1,2; speed of advance in, 2 

Morning Mine, Mullan, Idaho, compressor plant at, 36, 38-40 

Mule haulage, cost of, 306, 309, 312 

Mushroom valve, 94, 95, 100 

N 

"«," valves of, 48, 50, 51, 61, 89, 134, 137, 138, 142 

Neumann and Esser piston air valves, 132 

New York Air Compressor Co., 42 

New York Aqueduct, explosion in compressor, 177 

Nicholson, J. T., experiments by, 238 

Nominal and actual cut-offs in air engines, 214-216 

Non-conducting covering for air pipe, 236, 237, 238 

Nordberg compressor, 19, 42, 58, 59, 92; air-pressure regulator, 162, 163 

Norris, R. V., tests on compressed-air haulage plant, 311-313 

North Star Mine (Cal.), compressor at, 36; reheating at, 237 

Norwalk compressor, 2, 12, 19, 42, 75, 76, 92, 99, 300, 301, 306, 309, 311; inter- 
cooler, 76, 81; poppet inlet valve, 94; "skip-valve," 99, 100; pressure regu- 
lators, 152, 153, 154; receiver, 145 

Norwich (Conn.) hydraulic air compressor, 193 



Ohio Society of Mech., Elec, and Steam Engineers, Trans, of, 276 

Oil-cataract delivery valves, 112, 113 

Oils, lubricating, 171, 173, 174, 175, 177, 181; flash and ignition points, 173, 175; 

oxidation of, 172, 173, 175 
Operation of compressors: stage, 75-90; steam-driven, 28 
Output of compressors, 17, 39, 40, 63, 65, 67, 84, 86, 134-139, 167, 168, 188, 192 



Paris Pneumatic Supply Co., 84, 229, 231 
Partial or incomplete expansion in air engines, 212 
Peerless Colliery (W. Va.), compressed-air haulage at, 30^ 

21 



32 2 INDEX 

Pelton water-wheel, 32, 36, 37, 38 

Pefioles, Compania de, compressor at, 41 

Performance of air compressors, 133-143, 167, 188, 192 

Phila. and Reading C. and I. Co. compressed-air locomotives, 300, 308, 310 

Pipe line for compressed-air haulage, 294; calculations for, 295 

Pipe, nominal and actual diameters, 197, 201; bends in, 206; joints in, 205; 

leakage, 198, 205, 312, 313; precautions in laying. 205 
Piston air valves, 132 

Piston clearance in air-compressor cylinders, 62-66, 70, 92 
Piston clearance in air engines, 214-217 
Piston inlet valves, Ingersoll-Sergeant, 92, 101, 108 
Piston speed of compressors, 49, 53, 54, 55, 61, 71, 88, 93, 115 
Plymouth Cordage Co., compressed-air locomotive at, 283 
Pneumatic displacement pumps, 265-270; consumption of air by, 267, 268 
Pneumatic Engineering Co., 269, 270 
Pohle air-lift pump, 270-278 
Pohle, Dr. Julius, 270 

Pokorny and Wittekind piston air valve, 132 
Poppet valves, 91, 94-100; cam-controlled, 130, 131; inertia of, 93, 94; sticking 

of, 99 
Port area for air cylinders, 93, 102, 105, 114, 115 
Porter, H. K., Co., compressed-air locomotives, 283, 284-286, 289, 290, 292, 295, 

3°5» 3 o8 > 3 10 , 3 11 
Preheating for compressed-air pumps, 261, 263 

Prescott steam pump, 252 

Pressure of air as influencing freezing, 222, 223 

Pressure regulators, 150-163 

Prevention of freezing of moisture, 221, 222, 260, 263 

Proportions of compressor cylinders, 31,32 

Protection from freezing of surface air piping, 224 

Pumping by direct action of compressed air, 265-281 

Pumps, direct-acting, 250-252; compound pumps operated by air, 261-264; 

cylinder dimensions, 254; duty of, 256, 257; terminal and mean air pressures, 

257; volume of air consumed, 255-258 
Pumps operated by direct action of compressed air, 223, 250-264; air-lift pumps, 

270-281; pneumatic displacement pumps, 265-270 



Railway and Engineering Review, 184 

Rand compressor, 2, 3, 62, 91, 156; reheater, 233 

Rand mechanical air valve gear, 91 

Randall, B. M., experiments by, 274 

Randolph, B. S., 314 

Rating of compressors, 133, 134 

Rawhide pinions for geared compressors, 41 

Receiver aftercoolers, 146, 149 

Receivers, air, 144-149; baffle-plate for, 149; functions of, 145, 146; sizes of, 144; 

underground receivers, 147, 223, 259 
Reducing valves for compressed-air pump, 259 
Reduction of pressure as influencing deposition of moisture, 223 
Re-expansion line of air card, 14 
Regulation of compressors, 17, 19, 22, 28, 30 
Regulators, air-pressure, 150-163 
Reheaters for compressed air, 231-239 
Reheating compressed air, 225-239, 261-264, 294 
Resistance due to bends in air piping, 206 



INDEX 323 

"Return air" system of transmission, 218, 219, 264, 269 

Richards, Frank, 75, 78, 108, 149, 201, 202, 218, 226, 237, 254 

Riedler compressor, 12, 24, 25, 113, 114; "express" discharge valve, 113, 114; 
mechanically controlled valves, 126-130 

Riedler, experiments by, 230 

Riedler steam pump, 252 

Risdon water-wheel, 32, ^^, 34, 35 

Rix Compressor and Drill Co., 42 

Rix compressor, North Star Mine, 36; compressed-air locomotive, 306 

Rix, E. A., on compressed-air pumps, 258, 260-263, 2 ^8, 274 

Rock drills, air pressure for, 32, 245-247; consumption of air by, 242-247; re- 
heaters for, 236; valve motion of, 248 

Rockford, 111., tests on air-lift pump, 274 

Rose, A. H., 192 

Rose Deep Mine, South Africa, experiments on rock-drills at, 245-246 

Rubber ring inlet valve, 103, 104 



Saunders, W. L., 47, 63, 67, 70, 115, 175, 239 

Schneider & Co., Creusot, 91 

Schram intercooler, 84 

Schuechtermann and Kremer oil-cataract valve, 113 

Schuetz, G. A., Wurzen, air-cataract valve, 113 

Sergeant reheater, 232, 233 

Seymour, L. I., experiments on rock-drills, 245 

Shetucket River, Conn., hydraulic air compressor, 193 

Sibley Journal of Engineering, 233 

Single-stage compression, work of, 134, 135 

" Skip-valve " for Norwalk high-pressure air cylinder, 99, 100 

Slimes and sands pumped by air-lift, 277-279 

Soap and water for cleaning air cylinders, 181 

Sommeiller, 1; compressor, 2 

South African Association of Engineers, Trans, of, 245 

Specific heat of air: at constant pressure and volume, 48, 142 

Speer, F. W., 192 

Spiral-weld steel air pipe, 205 

Spool versus tappet valves for machine drills, 248 

Springs for inlet valves, 94; resistance of, 94-99, 107 

Stage compressors, 12, 16, 18, 19, 20, 21, 22, 23, 24-29, 71-89; double-acting, 75; 
for high altitudes, 169; ratio of cylinder volumes, 77-80; single-acting, 74; 
work of, 137, 138 

Steam and air card combined, 31 

Steam-driven, direct-acting pumps, 250-252; cylinder dimensions of, 254, 255 

Storage-battery electric locomotives, 283 

Straight-line compressors, 9, 12, 13, 17, 18, 19, 20 

Stroke, length of, for compressors, 31 

Sturgeon air inlet valve, 92, 108, 131 

Submergence of air-lift pumps, 274, 276-278 

Suction valves (see Inlet valves) 

Sullivan compressor, 12, 42, 81, 82, 124, 158, 233, 301; intercooler, 81, 82; re- 
heater, 233-235; valve motions, 124, 125 

Sullivan Machinery Co., 42, 301 

Summers. L. L., experiments bv. 245 

Surface air piping, protection of, 224 

Susquehanna Coal Co., No. 6 Colliery, compressed-air haulage at, 31 1-3 13 

Sutro tunnel, Nevada^ compressor at, 52 



324 • INDEX 



Tailings pumps and wheels replaced by air-lift pumps, 277-279 

Tanks, capacity of, for compressed-air locomotives, 295, 299, 305 et seq. 

Tappet versus spool valves for machine drills, 248 

Taylor, Chas. H., 184; hydraulic air compressor, 184-193, 238 

Technical Society of the Pacific Coast, Proceedings of, 258, 268, 274 

Temperatures, rate of increase of, in compression, 45 

Temperature, of compression, 40, 44, 46-51, 53, 55, 5 6 > 61, 62, 71, 140, 143, 171, 

178, 179, 228 
Temperatures of expansion, 210, 211 
Temperatures employed in reheating, 227-230, 237, 239 
Temple "electric-air" drill, 6 
Tennessee Copper Co., compressor of, 19 
Theoretical horse-power required to compress air, 134, 135 
Thermal cost of reheating, 226-230 
Three-stage compression, work of, 137 
Three-stage compressors, 72, 300, 302 
Track resistance of mine cars, 298, 299 

Transmission losses, comparison of air and steam, 4; in pipes, 194-206 
Transmission of power by compressed air, 194-206 
Two-pipe system of compressed-air transmission, 218, 219 
Two-stage compression, work of, 137 
Tunnelling, 1, 2, 5 
Turbine wheel for driving compressors, 32 

U 

Underground air receivers, 146, 147, 223, 259 
Underground reheaters, 236 

Unloaders for air cylinders, 156-162; Franklin, 159; Ingersoll-Sergeant, 159-161, 
Laidlaw-Dunn-Gordon, 161; Norwalk, 153; Rand, 156, 157; Sullivan, 158, 

159 
Unwin, W. C, 202, 230 



Valve, adjustable cut-off, for steam, 30 

Valves, air -inlet, 91-109; area of, 93, 102; of Bailey & Co., 132; chattering of, 
94, 96; clack, 91, 105; Corliss, 92, 117, 123-125; of Dover Iron Co., 132; 
Guttermuth, 91; Humboldt rubber-ring, 103, 104; inertia of, 93, 94; Inger- 
soll-Sergeant piston inlet, 101, 102; Johnson, 102, 103; Koster piston valve, 
132; Laidlaw-Dunn-Gordon, 95; Lens cam-controlled, 130, 131; Leyner 
flat annular, 105, 106, 107; mechanically controlled, 91, 92, 93, 116, 168; 
Neumann and Esser, 132; Nordberg, 42, 58, 92; Norwalk, 94; Pokorny and 
Wittekind, 132; requisites of, 92; resistance of, 94-99, 107, 168; Riedler 
double seat poppet, 126, 128, 130; Schuechtermann and Kremer, 113; Schuetz, 
113; skip-valve, 99, 100, 101, 105, 107; springs for, 93, 94, 95, 96, 97; sticking 
of, 99; Sturgeon, 131 

Valves, air-delivery, 110-115; air-cataract, 113; Allis-Chalmers, 123; area of, 114, 
115; Corliss, 120, 122, 123; Laidlaw-Dunn-Gordon "Cincinnati" valve 
gear, 113; poppet, 121-123; Nordberg, 118, 120; Norwalk, 118, 119; oil- 
cataract, 112, 113; Riedler "express," 113, 114; double-seat poppet, 126, 129, 
130 

Valves of machine drills, 248 



INDEX 325 

Van Nostrand's Science Series No. 106, Unwin, 202 

Vauclain compound compressed-air locomotive, 310 

Velocity of flow of air in pipes, 180, 202-205 

Victoria Copper Mine, Mich., hydraulic air compressor at, 190-193 

Volume of air for non-expansive working pumps, 255-258 

Volumes and pressures of compressed air, 133, 134, 139; at altitudes above sea-level, 

164-169; influence of reheating on, 226, 228-230, 238 
Vulcan Iron Works, 42 f 

W 

Wainwright water heater employed as reheater, 263 

Wandsworth (England) test on air-lift pump, 276 

Water-driven compressors, 32, 34, 35, 36, 37-40; at Goleta Mine, 32; at Morning 

Mine, 36, 38-40; at North Star Mine, 36, 38, 39 
Water-wheels for driving compressors: Knight, 31; Pelton, 32, 35; peripheral 

velocity of, 32; Risdon, 32, SS, 34 
Weber, F. C., 216 
Weight and volume of dry air, 44 
Wet compressors, 50, 51, 52-57, 66-70 
"Wet" reheating, 238, 239 
Weymouth, C. R., 239 
Wilson tests on air-lift pumps, 277-279 
Wilson Colliery, Pa., compressed-air haulage at, 309 
Woodbridge, D. E., 192 
Work done by air engines, 21 1-2 16 
Work done by air-lift pumps, 275 
Work gained by reheating, 227-230 

Work required to compress air, 68, 86, 134, 135; in stage compression, 137, 138 
Worthington, Henry R., 250 
Worthington compound pump at Gwin Mine, 263 



Zahner, "Transmission of Power by Compressed Air," 46, 55, 56, 210 
Zeitschrift fur das Berg-, Hiitten- und Salinen-Wesen, 147 



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Part I. Propagation, Culture, and Improvement i2mo, 1 50 

Part II. Systematic Pomology nmo, 1 50 

Elliott's Engineering for Land Drainage i2mo, 1 50 

Practical Farm Drainage 2d Edition, Rewritten . nmo, 1 50 

Graves's Forest Mensuration _ , . . 8vo, 4 00 

Green's Principles of American Forestry , . nmo, 1 50 

Grotenfelt's Principles of Modern Dairy Practice. (Woll) nmo, 2 00 

* Herrick's Denatured or Industrial Alcohol 8vo, 4 00 

Kemp and Waugh's Landscape Gardening. New Edition, Rewritten. (In 

Preparation.) 

* McKay and Larsen's Principles and Practice of Butter-making 8vo, 1 50 

Maynard's Landscape Gardening as Applied to Home Decoration nmo, 1 50 

Quaintance and Scott's Insects and Diseases of Fruits. (In Preparation). 

Sanderson's Insects Injurious to Staple Crops nmo, 1 50 

* Schwarz's Longleaf Pine in Virgin Forests nmo, 1 25 

Stockbridge's Rocks and Soils 8vo, 2 50 

Winton's Microscopy of Vegetable Foods 8vo, 7 50 

Woll's Handbook for Farmers and Dairymen i6mo, 1 50 

ARCHITECTURE. 

Baldwin's Steam Heating for Buildings nmo, 2 50 

Berg's Buildings and Structures of American Railroads 4to, 5 00 

Birkmire's Architectural Iron and Steel 8vo, 3 50 

Compound Riveted Girders as Applied in Buildings 8vo, 2 00 

Planning and Construction of American Theatres 8vo, 3 00 

Planning and Construction of High Office Buildings 8vo, 3 50 

Skeleton Construction in Buildings. 8vo, 3 00 

Briggs's Modern American School Buildings 8vo, 4 00 

Byrne's Inspection of Material and Wormanship Employed in Construction. 

i6mo, 3 00 

Carpenter's Heating and Ventilating of Buildings 8vo, 4 00 

* Cor their s Allowable Pressure on Deep Foundations nmo, 1 25 

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Freitag's Architectura 1 Engineering 8vo 

Fireproofing of Steel Buildings 8vo, 

French and Ives's Stereotomy 8vo, 

Gerhard's Guide to Sanitary House-Inspection i6mo, 

* Modern Baths and Bath Houses 8vo, 

Sanitation of Public Buildings i2mo, 

Theatre Fires and Panics i2mo, 

Holley and Ladd's Analysis of Mixed Paints, Color Pigments, and Varnishes 

Large nmo, 

Johnson's Statics by Algebraic and Graphic Methods 8vo, 

Kellaways How to Lay Out Suburban Home Grounds 8vo, 

Kidder's Architects' and Builders' Pocket-book i6mo, mor. 

Maire's Modern Pigments and their Vehicles i2mo, 

Merrill's Non-metallic Minerals: Their Occurrence and Uses 8vo, 

Stones for Building and Decoration .8vo, 

Monckton's Stair-building 4to, 

Patton's Practical Treatise on Foundations 8vo, 

Peabody's Naval Architecture 8vo, 

Rice's Concrete-block Manufacture 8vo, 

Richey's Handbook for Superintendents of Construction i6mo, mor. 

* Building Mechanics' Ready Reference Book: 

* Building Foreman's Pocket Book and Ready Reference. (In 

Preparation.) 

* Carpenters' and Woodworkers' Edition i6mo, mor. 

* Cement Workers and Plasterer's Edition i6mo, mor. 

* Plumbers', Steam-Filters', and Tinners' Edition .... i6mo, mor. 

* Stone- and Brick-masons' Edition i6mo, mor. 

Sabin's House Painting nmo, 

Industrial and Artistic Technology of Paints and Varnish 8vo, 

Siebert and Biggin's Modern Stone-cutting and Masonry 8vo, 

Snow's Principal Species of Wood 8vo, 

Towne's Locks and Builders' Hardware i8mo, mor. 

Wait's Engineering and Architectural Jurisprudence 8vo, 

Sheep, 

Law of Contracts 8vo, 

Law of Operations Preliminary to Construction in Engineering and Archi- 
tecture - - 8vo, 

Sheep, 

Wilson's Air Conditioning nmo, 

Worcester and Atkinson's Small Hospitals, Establishment and Maintenance, 
Suggestions for Hospital Architecture, with Plans for a Small Hospital. 

i2mo, 1 25 

ARMY AND NAVY. 

Bernadou's Smokeless Powder, Nitro-cellulose, and the Theory of the Cellulose 

Molecule nmo, 

Chase's Art of Pattern Making nmo, 

Screw Propellers and Marine Propulsion 8vo, 

Cloke's Enlisted Specialist's Examiner. (In Press.) 

Gunner's Examiner 8vo, 

Craig's Azimuth 4to, 

Crehore and Squier's Polarizing Photo-chronograph 8vo. 

* Davis's Elements of Law 8vo, 

* Treatise on the Military Law of United States 8vo, 

Sheep, 
De Brack's Cavalry Outpost Duties. (Carr) 24mo, mor. 

* Dudley's Military Law and the Procedure of Courts-martial.. . Large nmo, 
Durand's Resistance and Propulsion of Ships 8vo, 

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* Dyer's Handbook of Light Artillery l2mo, 

Eissler's Modern High Explosives .8vo, 

* Fiebeger's Text-book on Field Fortification Large i2mo, 

Hamilton and Bond's The Gunner's Catechism i8mo, 

* Hoff's Elei lentary Naval Tactics 8vo, 

Ingalls's Handbook of Problems in Direct Fire 8vo, 

* Lissak's Ordnance and Gunnery 8vo, 

* Ludlow's Logarithmic and Trigonometric Tables 8vo, 

* Lyons's Treatise on Electromagnetic Phenomena. Vols. I. and II.. 8vo, each, 

* Mahan's Permanent Fortifications. (Mercur) 8vo, half mor. 

Manual for Courts-martial i6mo, mor. 

* Mercur's Attack of Fortified Places i2mo, 

* Elements of the Art of War 8vo, 

Metcalf's Cost of Manufactures — And the Administration of Workshops. .8vo, 

Nixon's Adjutants' Manual 24mo, 

Peabody's Naval Architecture 8vo, 

* Phelps's Practical Marine Surveying 8vo, 

Putnam's Nautical Charts. (In Press.) 

Sharpe's Art of Subsisting Armies in War i8mo, mor. 1 50 

* Tupes and Poole's Manual of Bayonet Exercises and Musketry Fencing. 

24010, leather, 50 

* Weaver's Military Explosives 8vo, 3 00 

Woodhull's Notes on Military Hygiene iomo, 1 50 



ASSAYING. 

Betts's Lead Refining by Electrolysis 8vo, 4 00 

Fletcher's Practical Instructions in Quantitative Assaying with the Blowpipe. 

i6mo, mor. 

Furman's Manual of Practical Assaying 8vo, 

Lodge's Notes on Assaying and Metallurgical Laboratory Experiments. . . .8vo, 

Low's Technical Methods of Ore Analysis 8vo, 

Miller's Cyanide Process i2mo, 

Manual of Assaying nmo, 

Minet's Production of Aluminum and its Industrial Use. (Waldo) nmo, 

O'Driscoll's Notes on the Treatment of Gold Ores 8vo, 

Ricketts and Miller's Notes on Assaying 8vo, 

Robine and Lenglen's Cyanide Industry. (Le Clerc) 8vo, 

Hike's Modern Electrolytic Copper Refining 8vo, 

Wilson's Chlorination Process nmo, 

Cyanide Processes nmo, 



ASTRONOMY. 

Comstock's Field Astronomy for Engineers 8vo, 

Craig's Azimuth 4to, 

Crandall's Text-book on Geodesy and Least Squares 8vo, 

Doolittle's Treatise on Practical Astronomy 8vo, 

Gore's Elements of Geodesy 8vo, 

Hayford's Text-book of Geodetic Astronomy 8vo, 

Merriman's Elements of Precise Surveying and Geodesy 8vo, 

* Michie and Harlow's Practical Astronomy 8vo, 

Rust's Ex-meridian Altitude, Azimuth and Star-Finding Tables. (In Press.) 

* White's Elements^of Theoretical and Descriptive. Astronomy nmo, 2 00 

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CHEMISTRY. 

* Abderhalden's Physiological Chemistry in Thirty Lectures. (Hall and Defren) 

8vo, 5 oo 

* Abegg's Theory of Electrolytic Dissociation, (von Ende) nmo, i 25 

Alexeyeff'c General Principles of Organic Syntheses. (Matthews) 8vo, 3 00 

Allen's Tables for iron Analysis 8vo, 3 00 

Arnold's Compendium of Chemistry. (Mandel) Large nmo, 3 50 

Association of State and National Food and Dairy Departments, Hartford, 

Meeting, 1906 8vo, 3 00 

Jamestown Meeting, 1907 8vo, 3 00 

Austen's Notes for Chemical Students i2mo, 1 50 

Baskerville's Chemical Elements. (In Preparation.) 

Bernadou's Smokeless Powder. — Nitro-cellulose, and Theory of the Cellulose 

Molecule nmo, 2 50 

* Blanchard's Synthetic Inorganic Chemistry nmo, 1 co 

* Browning's Introduction to the Rarer Elements 8vo, 1 50 

Brush and Penfield's Manual of Determinative Mineralogy 8vo, 4 00 

* Claassen's Beet-sugar Manufacture. (Hall and Rolfe) 8vo, 3 00 

Classen's Quantitative Chemical Analysis by Electrolysis. (Boltwood). . .8vo, 3 00 

Cohn's Indicators and Test-papers nmo, 2 00 

Tests and Reagents 8vo, 3 00 

* Danneel's Electrochemistry. (Merriam) nmo, 1 25 

Duhem's Thermodynamics and Chemistry. (Burgess) 8vo, 4 00 

Eakle's Mineral Tables for the Determination of Minerals by their Physical 

Properties 8vo, 1 25 

Eissler's Modern High Explosives 8vo, 4 00 

Effront's Enzymes and their Applications. (Prescott) 8vo ; 3 00 

Erdmann's Introduction to Chemical Preparations. (Dunlap) nmo, 1 25 

* Fischer's Physiology of Alimentation Large nmo, 2 00 

Fletcher's Practical Instructions in Quantitative Assaying with the Blowpipe. 

nmo, mor. 1 50 

Fowler's Sewage Works Analyses nmo, 2 00 

Fresenius's Manual of Qualitative Chemical Analysis. (Wells) 8vo, 5 00 

Manual of Qualitative Chemical Analysis. Part I. Descriptive. (Wells) 8vo, 3 00 

Quantitative Chemical Analysis. (Cohn) 2 vols 8vo, 12 50 

When Sold Separately, Vol. I, $6. Vol. II, $8. 

Fuertes's Water and Public Health nmo, 1 50 

Furman's Manual of Practical Assaying 8vo, 3 00 

* German's Exercises in Physical Chemistry nmo, 2 00 

Gill's Gas and Fuel Analysis for Engineers nmo, 1 25 

* Gooch and Browning's Outlines of Qualitative Chemical Analysis. 

Large nmo, 1 25 

Grotenfelt's Principles of Modern Dairy Practice. (Woll) nmo, 2 00 

Groth's Introduction to Chemical Crystallography (Marshall) nmo, 1 25 

Hammarsten's Text-book of Physiological Chemistry. (Mandel) 8vo, 4 00 

Hanausek's Microscopy of Technical Products. Winton) 8vo, 5 00 

* Haskins and Macleod's Organic Chemistry i2mo, 2 00 

Helm's Principles of Mathematical Chemistry. (Morgan) nmo, 1 50 

Hering's Ready Reference Tables (Conversion Factors) i6mo, mor. 2 50 

* Herrick's Denatured or Industrial Alcohol 8vo, 4 00 

Hinds's Inorganic Chemistry 8vo, 3 00 

* Laboratory Manual for Students nmo, 1 00 

* Holleman's Laboratory Manual of Organic Chemistry for Beginners. 

(Walker) nmo, 1 00 

Text-book of Inorganic Chemistry. (Cooper) 8vo, 2 50 

Text-book of Organic Chemistry. (Walker and Mott) 8vo, 2 50 

Holley and Ladd's Analysis of Mixed Paints, Color Pigments , and Varnishes. 

Large nmo, 2 50 
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Hopkins's Oil-chemists' Handbook .8vo, 

Iddings's Rock Minerals 8vo , 

Jackson's Directions for Laboratory Work in Physiological Chemistry. .8vo, 
Johannsen's Determination of Rock-forming Minerals in Thin Sections. . .8vo, 
Johnson's Chemical Analysis of Special Steels. (In Preparation.) 

Keep's Cast Iron 8vo, 

Ladd's Manual of Quantitative Chemical Analysis i2mo, 

Landauer's Spectrum Analysis. (Tingle) 8vo, 

* Langworthy and Austen's Occurrence of Aluminium in Vegetable Prod- 

ucts, Animal Products, and Natural Waters 8vo, 

Lassar-Cohn's Application of Some General Reactions to Investigations in 

Organic Chemistry. (Tingle) i2mo, 

Leach's Inspection and Analysis of Food with Special Reference to State 

Control 8vo, 

Lob's Electrochemistry of Organic Compounds. (Lorenz) 8vo, 

Lodge's Notes on Assaying and Metallurgical Laboratory Experiments. .. .8vo, 

Low's Technical Method of Ore Analysis 8vo, 

Lunge's Techno-chemical Analysis. (Conn)..' 1 i2mo, 

* McKay and Larsen's Principles and Practice of Butter-making ...... .8vo, 

Maire's Modern Pigments and their Vehicles i2mo, 

Mandel's Handbook for Bio-chemical Laboratory i2mo, 

* Martin's Laboratory Guide to Qualitative Analysis with the Blowpipe . . i2mo, 
Mason's Examination of Water. (Chemical and Bacteriological.). . . .nmo, 

Water-supply. (Considered Principally from a Sanitary Stan dpi 

8vo, 
Matthews's Textile Fibres. 2d Edition, Rewritten 8vo, 

* Meyer's Determination of Radicles in Carbon Compounds. (Tingle). . nmo, 
Miller's Cyanide Process i2mo, 

Manual of Assaying i2mo, 

Minet's Production of Aluminum and its Industrial Use. (Waldo) i2mo, 

Mixter's Elementary Text-book of Chemistry i2mo, 

Morgan's Elements of Physical Chemistry i2mo, 

Outline of the Theory of Solutions and its Results i2mo, 

* Physical Chemistry for Electrical Engineers i2mo, 

Morse's Calculations used in Cane-sugar Factories i6mo, mor. 

* Muir's History of Chemical Theories and Laws 8vo, 

Mulliken's General Method for the Identification of Pure Organic Compounds. 

Vol. I Large 8vo, 

O'Driscoll's Notes on the Treatment of Gold Ores 8vo, 

Ostwald's Conversations on Chemistry. Part One. (Ramsey) i2mo, 

Part Two. (Turnbull) i 2 mo, 

* Palmer's Practical Test Book of Chemistry i2nio, 

* Pauli's Physical Chemistry in the Service of Medicine. (Fischer^ nmo, 

* Penfield's Notes on Determinative Mineralogy and Record of Mineral Tests. 

8vo, paper, 50 
Tables of Minerals, Including the Use of Minerals and Statistics of 

Domestic Production 8vo, 1 00 

Pictet's Alkaloids and their Chemical Constitution. (Biddle) 870, 5 00 

Poole's Calorific Power of Fuels 8vo, 3 00 

Prescott and Winslow's Elements of Water Bacteriology, with Special Refer- 
ence to Sanitary Water Analysis i2mo, 1 50 

* Reisig's Guide to Piece-dyeing 8vo, 25 00 

Richards and Woodman's Air, Water, and Food from a Sanitary Standpoint. .8 vo , 2 00 

Ricketts and Miller's Notes on Assaying 8vo, 3 00 

Rideal's Disinfection and the Preservation of Food 8vo, 4 00 

Sewage and the Bacterial Purification of Sewage 8vo, 4 00 

Riggs's Elementary Manual for the Chemical Laboratory 8vo, 1 25 

Robine and Lenglen's Cyanide Industry. (Le Clerc) 8vo, 4 

Ruddiman's Incompatibilities in Prescriptions 8vo, 2 

Whys in Pharmacy i2mo, 1 oa 

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Ruer's Elements of Metallography. (Mathewson) (In Preparation.) 

Sabin's Industrial and Artistic Technology of Paints and Varnish 8vo, 3 00 

Salkowski's Physiological and Pathological Chemistry. (Orndorff) . 8vo, 2 50 

Schimpf's Essentials of Volumetric Analysis nmo, 1 25 

* Qualitative Chemical Analysis 8vo. r 25 

Text-book of Volumetric Analysis nmo, ? 50 

Smith's Lecture Notes on Chemistry for Dental Students . . 8vo v 2 50 

Spencer's Handbook for Cane Sugar Manufacturers i6mo, mor, 3 00 

Handbook for Chemists of Beet-sugar Houses i6mo, mor. 3 00 

Stockbridge's Rocks and Soils 8vo, 2 50 

* Tillman's Descriptive General Chemistry ,8vo, 3 00 

* Elementary Lessons in Heat 8vo v 1 50 

TreadwelTs Qualitative Analysis. (Hall) 8vo s 3 00 

Quantitative Analysis. (Hall) 8vo> 4 00 

Turneaure and Russell's Public Water-supplies 8vo, 5 00 

Van Deventer's Physical Chemistry for Beginners. (Boltwood) nmo, 1 50 

Venable's Methods and Devices for Bacterial Treatment of Sewage 8vo, 3 00 

Ward and Whipple's Freshwater Biology. (In Press.) 

Ware's Beet-sugar Manufacture and Refining. Vol. I Small 8vo, 4 00 

Vol.11 SmallSvo, 5 co 

Washington's Manual of the Chemical Analysis of Rocks 8vo, 2 00 

* Weaver's Military Explosives. . . .• 8vo, 3 00 

Wells's Laboratory Guide in Qualitative Chemical Analysis 8vo, r 50 

Short Course in Inorganic Qualitative Chemical Analysis for Engineering 

Students nmo, 1 50 

Text-book of Chemical Arithmetic nmo, 1 25 

Whipple's Microscopy of Drinking-water 8vo, 3 5c- 

Wilson's Chlorination Process .. . nmo, 1 53 

Cyanide Processes nmo, 1 50 

Winton's Microscopy of Vegetable Foods , 8vo, 7 5c 



CIVIL ENGINEERING. 

BRIDGES AND ROOFS. HYDRAULICS. MATERIALS OF ENGINEER- 
ING. RAILWAY ENGINEERING. 

Baker's Engineers' Surveying Instruments nmo, 3 00 

Bixby's Graphical Computing Table Paper iqt - 24! inches. 25 

Breed and Hosmer's Principles and Practice of Surveying. 2 Volumes. 

Vol. I. Elementary Surveying 8vo, 3 00 

Vol. II. Higher Surveying 8vo, 2 50 

* Burr's Ancient and Modern Engineering and the Isthmian Canal .... 8vo, 3 50 
Comstock's Field Astronomy for Engineers 8vo, 2 50 

* Corthell's Allowable Pressures on Deep Foundations nmo, 1 25 

Crandall's Text-book on Geodesy and Least Squares 8\-o, 3 00 

Davis's Elevation and Stadia Tables 8vo, 1 00 

Elliott's Engineering for Land Drainage nmo, 1 50 

Practical Farm Drainage nmo, t 00 

*Fiebeger's Treatise on Civil Engineering 8vo, 5 00 

Flemer's Phototopographic Methods and Instruments 8vo, 5 ,00 

Folwell's Sewerage. (Designing and Maintenance.) 8vo, 3 00 

Freitag's Architectural Engineering 8vo, 3 50 

French and Ives's Stereotomy 8vo, 2 50 

Goodhue's Municipal Improvements nmo, 1 50 

Gore's Elements of Geodesy. 8vo, 2 50 

* Hauch's and Rice's Tables of Quantities for Preliminary Estimates . . nmo, 1 25 

Hayford's Text-book of Geodetic Astronomy 8vo, 3 00 

Hering's Ready Reference Tables. (Conversion Factors) i6mo, mor. 2 50 

Howe's Retaining Walls for Earth nmo, 1 25 

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* Ives's Adjustments of the Engineer's Transit and Level i6mo, Bds. 25 

Ives and Hilts's Problems in Surveying i6mo, mor. 1 50 

Johnson's (J. B. ) Theory and Practice of Surveying Small 8vo, 4 00 

Johnson's (L. J.) Statics by Algebraic and Graphic Methods 8vo, 2 00 

Kinnicutt, Winslow and Pratt's Purification of Sewage. (In Preparation.) 
Laplace's Philosophical Essay on Probabilities. 'Truscott and Emory) 

i2mo, 2 00 

Mahan's Descriptive Geometry 8vo, 1 50 

Treatise on Civil Engineering. (1873.) (Wood) 8vo, 5 00 

Merriman's Elements of Precise Surveying and Geodesy 8vo, 2 50 

Merriman and Brooks's Handbook for Surveyors i6mo, mor. 2 00 

Nugent's Plane Surveying 8vo, 3 50 

Ogden's Sewer Construction. (In Press.) 

Sewer Design 1 2mo, 2 00 

Parsons's Disposal of Municipal Refuse 8vo, 2 00 

Patton's Treatise on Civil Engineering 8vo, half leather, 7 50 

Reed's Topographical Drawing and Sketching 4to, 5 00 

Rideal's Sewage and the Bacterial Purification of Sewage 8vo, 4 00 

Riemer's Shaft-sinking under Difficult Conditions. (Corning and Peele). . . 8vo, 3 00 

Siebert and Biggin's Modern Stone-cutting and Masonry 8vo, 1 50 

Smith's Manual of Topographical Drawing. (McMillan) 8vo, 2 50 

Soper's Air and Ventilation of Subways Large nmo, 2 50 

Tracy's Plane Surveying i6mo, mor. 3 00 

* Trautwine's Civil Engineer's Pocket-book i6mo, mor. 5 00 

Venable's Garbage Crematories in America 8vo, 2 00 

Methods and Devices for Bacterial Treatment of Sewage 8vo, 3 00 

Wait's Engineering and Architectural Jurisprudence 8vo, 6 00 

Sheep, 6 50 

Law of Contracts 8vo, 3 00 

Law of Operations Preliminary to Construction in Engineering and Archi- 
tecture j 8vo, 5 00 

Sheep, 5 50 

Warren's Stereotomy — Problems in Stone-cutting 8vo, 2 50 

* Waterbury's Vest-Pocket Hand-book of Mathematics for Engineers. 

' 2! X 5# inches, mor. 1 00 
Webb's Problems in the Use and Adjustment of Engineering Instruments. 

i6mo, mor. 1 25 

Wilson's (H. N.) Topographic Surveying 8vo. 3 50 

Wilson's (W. L.) Elements of Railroad Track and Construction. (In Press.) 

BRIDGES AND ROOFS, 



Boiler's Practical Treatise on the Construction of Iron Highway Bridges 8vo 
Burr and talk's Design and Construction of Metallic Bridges 8vo', 5 

Influence Lines for Bridge and Roof Computations 8vo' 3 

Du Bois's Mechanics of Engineering. Vol. n Srr all 4 to', 10 

Foster's Treatise on Wooden Trestle Bridges ^ + ' ~ 

Fowler's Ordinary Foundations o ' 

French and Ives's Stereotomy R ' ' 

Greene's Arches in Wood, Iron, and Stone 8 ' 

Bridge Trusses o ' 

RoofTrusses f°» 2 5 ° 

8vo, 1 2^ 

Gnmm's Secondary Stresses in Bridge Trusses g vo 

Heller's Stresses in Structures and the Accompanying Deformations 8vo' 

Howe's Design of Simple Roof-trusses in Wood and Steel 8vo' 

Symmetrical Masonry Arches o * 

Treatise on Arches ' 

T , _, ._ ovo, 4 00 

Johnson, Bryan, and Turneaure's Theory and Practice in the Designing of 

Modern Framed Structures c~,„n * 

omall 4to, 10 00 



2 00 
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Merriman and Jacoby's Text-book on Roofs and Bridges : 

Part I. Stresses in Simple Trusses 8vo, 2 50 

Part II. Graphic Statics . 8vo, 2 50 

Part III. Bridge Design 8vo, 2 50 

Part IV. Higher Structures 8vo, 2 50 

Morison's Memphis Bridge Oblong 4to, 10 00 

Sondericker's Graphic Statics, with Applications to Trusses, Beams, and Arches. 

8vo, 2 00 

WaddelTs De Pontibus, Pocket-book for Bridge Engineers. ..... i6mo, mor, 2 00 

* Specifications for Steel Bridges i2mo, 50 

Waddell and Harrington's Bridge Engineering. (In Preparation.) 

Wright's Designing of Draw-spans. Two parts in one volume 8vo, 3 50 



HYDRAULICS. 

Barnes's Ice Formation 8vo, 3 00 

Bazin's Experiments upon the Contraction of the Liquid Vein Issuing from 

an Orifice. (Trautwine) 8vo, 2 00 

Bovey's Treatise on Hydraulics 8vo, 5 00 

Church's Diagrams of Mean Velocity of Water in Open Channels. 

Oblong 4to, paper, 1 50 

Hydraulic Motors 8vo, 2 00 

Mechanics of Engineering 8vo, 6 00 

Coffin's Graphical Solution of Hydraulic Problems i6mo, mor. 2 50 

Flather's Dynamometers, and the Measurement of Power nmo, 3 00 

Folwell's Water-supply Engineering 8vo, 4 00 

Frizell's Water-power 8vo, 5 00 

Fuertes's Water and Public Health i2mo, 1 50 

Water-filtration Works i2mo, 2 50 

Ganguillet and Kutter's General Formula for the Uniform Flow of Water in 

Rivers and Other Channels. CHering and Trautwine; 8vo, 4 00 

Hazen's Clean Water and How to Get Tt Large nmo, 1 50 

Filtration of Public Water-supplies 8vo, 3 00 

Hazlehurst's Towers and Tanks for Water- works 8vo, 2 50 

Herschel's 115 Experiments on the Carrying Capacity of Large, Riveted, Metal 

Conduits 8vo , 2 00 

Hoyt and Grover's River Discharge 8vo, 2 00 

Hubbard and Kiersted's Water- works Management and Maintenance 8vo, 4 co 

* Lyndon's Development and Electrical Distribution of Wa'er Power. . . . 8vo, 3 oo 
Mason's Water-supply. (Considered Principally from a Sanitary Standpoint.) 

8vo, 4 00 

Merriman's Treatise on Hydraulics 8vo, 5 00 

* Michie's Elements of Analytical Mechanics 8vo, 4 00 

* Molitor's Hydraulics of Rivers. Wefts and Sluices . . Pvo, 2 00 

Schuyler's Reservoirs for Irrigation, Water-power, and Domestic Water- 
supply Large 8vo, 5 00 

* Thomas and Watt's Improvement of Rivers 4to, 6 00 

Turneaure and Russell's Public Water-supplies 8vo, 5 00 

Wegmann's Design and Construction of Dams. 5th Ed., enlarged ... 4+0, 6 00 

Water-supply of the City of New York from 1658 to 1895 4to, 10 00 

Whipple's Value of Pure Water Large i2mo, 1 00 

Williams and Hazen's Hydraulic Tables 8vo, 1 50 

Wilson's Irrigation Engineering Small 8vo, 4 00 

Wolff's Windmill as a Prime Mover 8vo, 3 00 

Wood's Elements of Analytical Mechanics 8vo, 3 00 

Turbines 8v0 - 2 50 

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MATERIALS OF ENGINEERING. 

Baker's Roads and Pavements 8vo, 

Treatise on Masonry Construction. 8vo, 

Birkmire's Architectural Iron and Steel Svo, 

Compound Riveted Girders as Applied in Buildings 8vo, 

Black's United States Public Works Oblong 4to, 

Bleininger's Manufacture of Hydraulic Cement. (In Preparation.) 

* Bovey's Strength of Materials and Theory of Structures 8vo, 

Burr's Elasticity and Resistance of the Materials of Engineering 8vo, 

Byrne's Highway Construction 8vo, 

Inspection of the Materials and Workmanship Employed in Construction. 

i6mo, 

Church's Mechanics of Engineering 8vo, 

Du Bois's Mechanics of Engineering. 

Vol. I. Kinematics, Statics, Kinetics Small 4to, 7 50 

Vol. II. The Stresses in Framed Structures, Strength of Materials and 

Theory of Flexures Small 4to, 10 00 

*Eckel's Cements, Limes, and Plasters Svo, 6 00 

Stone and Clay Products used in Engineering. (In Preparation.) 

Fowler's Ordinary Foundations 8vo, 3 50 

Graves's Forest Mensuration 8vo, 4 00 

Green's Principles of American Forestry nmo, 1 50 

* Greene's Structural Mechanics 8vo, 2 50 

Holly and Ladd's Analysis of Mixed Paints, Color Pigments and Varnishes 

Large i2mo, 2 50 
Johnson's (C. M.) Chemical Analysis of Special Steels. (In Preparation.) 

Johnson's (J. B.) Materials of Construction Large 8vo, 

Keep's Cast Iron 8vo y 

Kidder's Architects and Builders' Pocket-book i6mo, 

Lanza's Applied Mechanics 8vo, 

Maire's Modern Pigments and their Vehicles nmo, 

Martens's Handbook on Testing Materials. (Henning) 2 vols. 8vo, 

Maurer's Technical Mechanics 8vo, 

Merrill's Stones for Building and Decoration 8vo, 

Merriman's Mechanics of Materials 8vo, 

* Strength of Materials nmo, 

Metcalf's Steel. A Manual for Steel-users i2mo, 

Morrison's Highway Engineering 8vo, 

Patton's Practical Treatise on Foundations 8vo, 

Rice's Concrete Block Manufacture 8vo, 

Richardson's Modern Asphalt Pavements 8vo, 

Richey's Handbook for Superintendents of Construction i6mo, mor. 

* Ries's Clays: Their Occurrence, Properties, and Uses 8vo, 

Sabin's Industrial and Artistic Technology of Paints and Varnish 8vo, 

*Schwarz'sLongleafPinein Virgin Forest... I2mo ' 

Snow's Principal Species of Wood • 8v0 » 

Spalding's Hydraulic Cement ■ "mo, 

Text-book on Roads and Pavements 12m0 » 

Taylor and Thompson's Treatise on Concrete, Plain and Reinforced 8vo, 

Thurston's Materials of Engineering. In Three Parts 8vo, 

Part I. Non-metallic Materials of Engineering and Metallurgy 8vo, 

Part II. Iron and Steel 8v0 ' 

Part Til. A Treatise on Brasses, Bronzes, and Other Alloys and their 

Constituents 8v0 ' 

Tillson's Street Pavements and Paving Materials 8vo, 

Turneaure and Maurer's Principles of Reinforced Concrete Construction.. -8vo, 
Waterbury*s Manual of Instructionsfor the Use of Students in Cement Labora- 
tory Practice. (In Press.) 

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Wood's (De V.) Treatise on the Resistance of Materials, and an Appendix on 

the Preservation of Timber Svo, 2 00 

Wood's (M. P.) Rustless Coatings: Corrosion and Electrolysis of Iron and 

Steel Svo , 4 00 

RAILWAY ENGINEERING. 

Andrews's Handbook for Street Railway Engineers 3x5 inches, mor. 1 25 

Berg's Buildings and Structures of American Railroads 4to, 5 00 

Brooks's Handbook of Street Railroad Location. i6mo, mor. 1 50 

Butt's Civil Engineer's Field-book i6mo, mor. 2 50 

Crandall's Railway and Other Earthwork Tables. 8vo, 1 50 

Transition Curve i6mo, mor. 1 50 

* Crockett's Methods for Earthwork Computations 8vo, 1 50 

Dawson's "Engineering" and Electric Traction Pocket-book i6mo, mor. 5 00 

Dredge's History of the Pennsylvania Railroad: (1879. Paper, 5 00 

Fisher's Table of Cubic Yards Cardboard, 25 

Godwin's Railroad Engineers' Field-book and Explorers' Guide. . . i6mo, mor. 2 50 
Hudson's Tables for Calculating the Cubic Contents of Excavations and Em- 
bankments 8vo, 1 00 

Ives and Hilts'-:; Problems in Surveying, Railroad Surveying and Geodesy 

i6mo, mor. 1 50 

Molitor and Beard's Manual for Resident Engineers i6mo, 1 00 

Nagle's Fieid Manual for Railroad Engineers i6mo, mor. 3 00 

Philbrick's Field Manual for Engineers. i6rno, mor. 3 00 

Raymond's Railroad Engineering. 3 volumes. 

VoL I. Railroad Field Geometry. In Preparation.) 

VoL II. Elements of Railroad Engineering Svo, 3 50 

Vol. III. Railroad Engineer's Field Book. (In Preparation.) 

Searles's Field Engineering i6mo, mor. 3 00 

Railroad Spiral. i6mo, mor. 1 50 

Taylor's Prismoidal Formula* and Earthwork 8vo. 1 50 

* Trautwine's Field Practice of Laying Out Circular Curves for Railroads. 

i2mo. mor. 2 50 

* Method of Calculating the Cubic Contents of Excavations and Embank- 

ments by the Aid of Diagrams Svo, 2 00 

Webb's Economics of Railroad Construction Large nmo, 2 50 

Railroad Construction i6mo, mor. 5 00 

Wellington's Economic Theory of the Location of Railways Small 8vo, 5 00 

DRAWING. 

Barr's Kinematics of Machinery 8vo, 2 50 

* Bartlett's Mechanical Drawing 8vo, 3 00 

* " " " Abridged Ed. 8vo, 150 

Coolidge's Manual of Drawing 8vo paper, 1 00 

Coolidge and Freeman's Elements of General Drafting for Mechanical Engi- 
neers Oblong 4to, 2 50 

Durley's Kinematics of Machines 8vo, 4 00 

Emch's Introduction to Projective Geometry and its Applications 8vo, 2 50 

Hill's Text-book on Shades and Shadows, and Perspective 8vo, 2 00 

Jamison's Advanced Mechanical Drawing 8vo, 2 00 

Elements of Mechanical Drawing, 8vo, 2 50 

Jones's Machine Design: 

Part I. Kinematics of Machinery 8vo, 1 50 

Part H. Form, Strength, and Proportions of Parts. 8vo, 3 00 

MacCord's Elements of Descriptive Geometry 8vo, 3 oc 

Kinematics ; or, Practical Mechanism. 8vo, 5 00 

Mechanical Drawing , 4to, 4 00 

Velocity Diagrams 8vo, 1 50 

10 



McLeod's Descriptive Geometry Large i2mo, I 50 

* Mahan's Descriptive Geometry and Stone-cutting 8vo, 1 50 

Industrial Drawing. (Thompson) 8vo, 3 50 

Moyer's Descriptive Geometry 8vo, 2 00 

Reed's Topographical Drawing and Sketching 4to, 5 00 

Reid's Course in Mechanical Drawing 8vo, 2 00 

Text-book of Mechanical Drawing and Elementary Machine Design. 8vo, 3 00 

Robinson's Principles of Mechanism 8vo, 3 00 

Schwamb and Merrill's Elements of Mechanism 8vo* 3 00 

Smith's (R. S.) Manual of Topographical Drawing. (McMillan) 8vo, 2 50 

Smith (A. W.) and Marx's Machine Design 8vo, 3 00 

* Titsworth's Elements of Mechanical Drawing Oblong 8vo, 1 25 

Warren's Drafting Instruments and Operations nmo, * 1 23 

Elements of Descriptive Geometry, Shadows, and Perspective 8vo, 3 50 

Elements of Machine Construction and Drawing 8vo, 7 50 

Elements of Plane and Solid Free-hand Geometrical Drawing i2mo, 1 00 

General Problems of Shades and Shadows 8vo, 3 00 

Manual of Elementary Problems in the Linear Perspective of Form and 

Shadow i2mo, 1 00 

Manual of Elementary Projection Drawing nmo, 1 50 

Plane Problems in Elementary Geometry i2mo, 1 25 

Problems, Theorems, and Examples in Descriptive Geometry 8vo, 2 50 

Weisbach's Kinematics and Power of Transmission. (Hermann and 

Klein) 8vo, 5 00 

Wilson's (H. M.) Topographic Surveying 8vo, 3 50 

Wilson's (V. T.) Free-hand Lettering 8vo, 1 00 

Free-hand Perspective 8vo, 2 50 

Woolf's Elementary Course in Descriptive Geometry Large 8vo, 3 00 

ELECTRICITY AND PHYSICS. 

* Abegg's Theory of Electrolytic Dissociation, (von Ende) i2mo, 1 25 

Andrews's Hand-Book for Street Railway Engineering ... .3X5 inches, mor. 1 25 

Anthony and Brackett's Text-book of Physics. (Magie) Large i2mo, 3 00 

Anthony's Lecture-notes on the Theory of Electrical Measurements. . . . i2mo, 1 00 

Benjamin's History of Electricity 8vo, 3 00 

Voltaic Cell 8vo, 3 00 

Betts's Lead Refining and Electrolysis 8vo, 4 00 

Classen's Quantitative Chemical Analysis by Electrolysis. (Boltwood)..8vo, 3 00 

* Collins's Manual of Wireless Telegraphy i2mo, 1 50 

Mor. 2 00 

Crehore and Squier's Polarizing Photo-chronograph 8vo, 3 00 

* Danneel's Electrochemistry. (Merriam) ; nmo, 1 23 

Dawson's "Engineering" and Electric Traction Pocket-book .... i6mo, mor. 5 00 
Dolezalek's Theory of the Lead Accumulator (Storage Battery), (von Ende) 

i2mo, 2 50 

Duhem's Thermodynamics and Chemistry. (Burgess) 8vo, 4 00 

Flather's Dynamometers, and the Measurement of Power .nmo, 3 00 

Gilbert's De Magnete. (Mottelay). .' 8vo, 2 50 

* Hanchett's Alternating Currents nmo, 1 00 

Hering's Ready Reference Tables (Conversion Factors) i6mo, mor. 2 50 

* Hobart and Ellis's High-speed Dynamo Electric Machinery 8vo, 6 00 

Holman's Precision of Measurements 8vo, 2 00 

Telescopic Mirror-scale Method, Adjustments, and Tests .... Large 8vo , 73 

* Karapetoff's Experimental Electrical Engineering 8vo, 6 00 

Kinzbrunner's Testing of Continuous-current Machines. 8vo, 2 00 

Landauer's Spectrum Analysis. (Tingle) 8vo, 3 00 

Le Chatelier's High-temperature Measurements. (Boudouard — Burgess). .nmo, 3 00 

Lob's Electrochemistry of Organic Compounds. (Lorenz) 8vo, 3 00 

* London's Development and Electrical Distribntion of Water Tower . . . .8vo, 3 00 

11 



* Lyons's Treatise on Electromagnetic Phenomena. Vols. I. and II. 8vo, each, 

* Michie's Elements of Wave Motion Relating to Sound and Light 8vo, 

Morgan's Outline of the Theory of Solution and its Results nmo, 

* Physical Chemistry for Electrical Engineers i2mo, 

Niaudet's Elementary Treatise on Electric Batteries. (Fishback). . . .i2mo, 

* Norris's Introduction to the Study of Electrical Engineering 8vo, 

* Parshall and Hobart's Electric Machine Design 4to, half mor. 

Reagan's Locomotives: Simple, Compound, and Electric. New Edition. 

Large nmo, 

* Rosenberg's Electrical Engineering. (Haldane Gee — Kinzbrunner). .. .8vo, 

Ryan, Norris, and Hoxie's Electrical Machinery. Vol. 1 8vo, 

Swapper's Laboratory Guide for Students in Physical Chemistry i2rao, 

* Tillman's Elementary Lessons in Heat 8vo, 

Tory and Pitcher's Manual of Laboratory Physics Large nmo, 

Ulke's Modern Electrolytic Copper Refining ,. 6 8vo, 

LAW. 

* Davis's Elements ot Law , 8vo, 

* Treatise on the Military Law of United States 8vo, 

* Sheep, 

* Dudley's Military Law and the Procedure of Courts-martial . . . .Large 12 mo, 

Manual for Courts-martial. ._ i6mo, rnor. 

Wait's Engineering and Architectural Jurisprudence t 8vo, 

Sheep, 

Law of Contracts 8vo, 

Law of Operations Preliminary to Construction in Engineering and Archi- 
tecture 8vc 

Sheep, 

MATHEMATICS. 

Baker's Elliptic Functions 8vo, 

Briggs's Elements of Plane Analytic Geometry. (Bocher) nmo, 

* Buchanan's Plane and Spherical Trigonometry 8vo, 

Byerley's Harmonic Functions 8vo, 

Chandler's Elements of the Infinitesimal Calculus 12 mo, 

Compton's Manual of Logarithmic Computations nmo, 

* Dickson's College Algebra Large nmo, 

* Introduction to the Theory of Algebraic Equations Large nmo, 

Emch's Introduction to Projective Geometry and its Applications 8vo, 

Fiske's Functions of a Complex Variable 8vo, 

Halsted's Elementary Synthetic Geometry 8vo, 

Elements of Geometry 8vo, 

* Rational Geometry nmo, 

Hyde's Grassmann's Space Analysis 8vo, 

* Jonnson's (,J- B.) Three-place Logarithmic Tables: Vest-pocket size, paper, 

100 copies, 

* Mounted on heavy cardboard, 8X10 inches, 

10 copies, 

Johnson's (W. W.) Abridged Editions of Differential and Integral Calculus 

Large 12 mo, 1 vol. 

Curve Tracing in Cartesian Co-ordinates nmo, 

Differential Equations 8vo, 

Elementary Treatise on Differential Calculus Large nmo, 

Elementary Treatise on the Integral Calculus Large nmo, 

* Theoretical Mechanics 1 2mo, 

Theory of Errors and the Method of Least Squares .nmo, 

Treatise on Differential Calculus Large nmo, 

Treatise on the Integral Calculus Large nmo, 

Treatise on Ordinary and Partial Differential Equations. . Large nmo, 

12 



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Laplace's Philosophical Essay on Probabilities. (Truscott and Emory). .nmo, 2 00 

* Ludlow and Bass's Elements of Trigonometry and Logarithmic and Other 

Tables 8vo, 3 00 

Trigonometry and Tables published separately Each, 2 00 

* Ludlow's Logarithmic and Trigonometric Tables 8vo, 1 00 

Macfarlane's Vector Analysis and Quaternions 8vo, 1 00 

McMahon's Hyperbolic Functions 8vo, 1 00 

Manning's Irrational lumbers and their Representation by Sequences and 

Series nmo, 1 25 

Mathematical Monographs. Edited by Mansfield Merriman and Robert 

S. Woodward. . , Octavo, each 1 00 

No. 1. History of Modern Mathematics, by David Eugene Smith. 
No. 2. Synthetic Projective Geometry, by George Bruce Halsted. 
No. 3. Determinants, by Laenas Gifford Weld. No. 4. Hyper- 
bolic Functions, by James McMahon. No. 5. Harmonic Func- 
tions, by William E. Byerly. No. 6. Grassmann's Space Analysis, 
by Edward W. Hyde. No. 7. Probability and Theory of Errors, 
by Robert S. Woodward. No. 8. Vector Analysis and Quaternions, 
by Alexander Macfarlane. No. 9. Differential Equations, by 
William Woolsey Johnson. No. 10. The Solution of Equations, 
by Mansfield Merriman. No. 11. Functions of a Complex Variable, 
by Thomas S. Fiske. 

Maurer's Technical Mechanics 8vo, 4 00 

Merriman's Method of Least Squares 8vo, 2 00 

Solution of Equations 8vo, 1 00 

Rice and Johnson's Differential and Integral Calculus. 2 vols, in one. 

Large i2mo, 1 50 

Elementary Treatise on the Differential Calculus -Large i2mo, 3 00 

Smith's History of Modern Mathematics 8vo, 1 00 

* Veblen and Lennes's Introduction to the Real Infinitesimal Analysis of One 

Variable 8vo, 2 00 

* Waterbury's Vest Pocket Hand-Book of Mathematics for Engineers. 

2 iX 5 1 inches, mor. 1 00 

Weld's Determinations 8vo, 1 co 

Wood's Elements of Co-ordinate Geometry 8vo, 2 00 

Woodward's Probability and Theory of Errors 8vo, 100 

MECHANICAL ENGINEERING. 
MATERIALS OF ENGINEERING, STEAM-ENGINES AND BOILERS. 

Bacon's Forge Practice i2mo, 1 50 

Baldwin's Steam Heating for Buildings i2mo, 2 5c 

Bair's Kinematics of Machinery 8vo, 2 50 

* Bartlett's Mechanical Drawing 8vo, 3 00 

* " " " Abridged Ed 8vo, 150 

Benjamin's Wrinkles and Recipes i2mo, 2 00 

* Burr's Ancient and Modern Engineering and the Isthmian Canal 8vo, 3 50 

Carpenter's Experimental Engineering 8vo, 6 00 

Heating and Ventilating Buildings 8vo, 4 00 

Clerk's Gas and Oil Engine Large i2mo, 4 00 

Compton's First Lessons in Metal Working i2mo, 1 50 

Compton and De Groodt's Speed Lathe i2mo 1 

Coolidge's Manual of Drawing 8vo, paper, 1 

Coolidge and Freeman's Elements of General Drafting for Mechanical En- 
gineers Oblong 4to, 2 50 

Cromwell's Treatise on Belts and Pulleys i2mo, 1 50 

Treatise on Toothed Gearing i2mo, 1 50 

Durley's Kinematics of Machines 8vo, 4 00 

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Flather's Dynamometers and the Measurement of Power i2mo, 

Rope Driving , i2me, 

Gill's Gas and Fuel Analysis for Engineers i2mo, 

Goss'i Locomotive Sparks 8vo, 

Greene's Pumping Machinery. (In Preparation.) 

Hering's Ready Reference Tables (Conversion Factors) i6mo, mor. 

* Hobart and Ellis's High Speed Dynamo Electric Machinery 8vo. 

Hutton's Gas Engine. Svo, 

Jamison's Advanced Mechanical Drawing 8vo, 

Elements of Mechanical Drawing 8vo, 

Jones's Machine Design: 

Part I. Kinematics of Machinery .8vo, 

. Part II. Form, Strength, and Proportions of Parts 8vo, 

Kent's Mechanical Engineers' Pocket-book i6mo, mor. 

Kerr's Power and Power Transmission 8vo, 

Leonard's Machine Shop Tools and Methods' 8vo, 

* Lorenz's Modern Refrigerating Machinery. (Pope, Haven, and Dean") . . . 8vo, 
MacCord's Kinematics; or, Practical Mechanism 8vo, 

Mechanical Drawing 4to, 

Velocity Diagrams 8vo, 

MacFarland's Standard Reduction Factors for Gases 8vo, 

Mahan's Industrial Drawing. (Thompson) 8vo, 

* Parshall and Hobart's Electric Machine Design Small 4to, half leather, 12 

Peele's Compressed Air Plant for Mines Svo, 

Poole's Calorific Power of Fuels 8vo, 

* Porter's Engineering Reminiscences, 1855 to 1882 8vo, 

Reid's Course in Mechanical Drawing 8vo, 

Text-book of Mechanical Drawing and Elementary Machine Design. 8vo, 

Richard's Compressed Air i2mo, 

Robinson's Principles of Mechanism 8vo, 

Schwamb and Merrill's Elements of Mechanism 8vo, 

Smith's (0.) Press- working of Metals 8vo, 

Smith (A. W.) and Marx's Machine Design 8vo, 

Sorel ' s Carbureting and Combustion in Alcohol Engines . ("Woodward and Preston) . 

Large 12 mo, 

Thurston's Animal as a Machine and Prime Motor, and the Laws of Energetics. 

i2mo, 
Treatise on Friction and Lost Work in Machinery and Mill Work... 8vo, 

Tillson's Complete Automobile Instructor i6mo, 

mor. 

* Titsworth's Elements of Mechanical Drawing Oblong 8vo, 

Warren's Elements of Machine Construction and Drawing 8vo, 

* Waterbury's Vest Pocket Hand Book of Mathematics for Engineers. 

2IX5I inches, mor. 1 00 
Weisbach's Kinematics and the Power of Transmission. (Herrmann — 

Klein) 8vo, 5 00 

Machinery of Transmission and Governors. (Herrmann — Klein).. .8vo, 5 00 
Wood's Turbines. , 8vo, 2 50 

MATERIALS OF ENGINEERING. 

* Bovey's Strength of Materials and Theory of Structures 8vo, 

Burr's Elasticity and Resistance of the Materials of Engineering 8vo, 

Church's Mechanics of Engineering 8vo, 

* Greene's Structural Mechanics Svo, 

Holley and Ladd's Analysis of Mixed Paints, Color Pigments, and Varnishes. 

Large nmo, 

Johnson's Materials of Construction „ 8vo, 

Keep's Cast Iron 8vo, 

Lanza's Applied Mechanics 8vo., 

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Maire's Modern Pigments and their Vehicles nmo, 

Martens 's Handbook on Testing Materials. (Henning) 8vo, 

Maurer's Technical Mechanics 8vo, 

Merriman's Mechanics of Materials 8vo, 

* Strength of Materials nmo, 

Metcalf's Steel. A Manual for Steel-users nmo, 

Sabin's Industrial and Artistic Technology of Paints and Varnish 8vo, 

Smith's Materials of Machines nmo, 

Thurston's Materials of Engineering 3 vols., 8vo, 

Part I. Non-metallic Materials of Engineering and Metallurgy . . .8vo, 

Part II. Iron and Steel 8vo, 

Part III. A Treatise on Brasses, Bronzes, and Other Alloys and their 

Constituents 8vo, 

Wood's (De V.) Elements of Analytical Mechanics 8vo, 

Treatise on the Resistance of Materials and an Appendix on the 

Preservation of Timber 8vo, 2 00 

Wood's (M. P.) Rustless Coatings: Corrosion and Electrolysis of Iron and 

Steel 8vo, 4 00 



STEAM-ENGINES AND BOILERS. 

Berry's Temperature-entropy Diagram nmo, 

Carnot's Reflections on the Motive Power of Heat. (Thurston) nmo, 

Chase's Art of Pattern Making nmo, 

Creighton's Steam-engine and other Heat-motors 8vo, 

Dawson's "Engineering" and Electric Traction Pocket-book i6mo, mor. 

Ford's Boiler Making for Boiler Makers i8mo, 

Gebhardt's Steam Power Plant Engineering. (In Press.) 

Goss's Locomotive Performance 8vo, 

Hemenway's Indicator Practice and Steam-engine Economy nmo, 

Hutton's Heat and Heat-engines 8vo, 

Mechanical Engineering of Power Plants 8vo, 

Kent's Steam boiler Economy 8vo, 

Kneass's Practice and Theory of the Injector 8vo, 

MacCord's Slide-valves 8vo, 

Meyer's Modern Locomotive Construction 4to, 

Moyer's Steam Turbines. (Tn Press.) 

Peabody's Manual of the Steam-engine Indicator nmo. 

Tables of the Properties of Saturated Steam and Other Vapors 8vo, 

Thermodynamics of the Steam-engine and Other Heat-engines 8vo, 

Valve-gears for Steam-engines 8vo, 

Peabody and Miller's Steam-boilers 8vo, 

Pray's Twenty Years with the Indicator Large 8vo, 

Pupin's Thermodynamics of Reversible Cycles in Gases and Saturated Vapors. 

(Osterberg) nmo, 1 25 

Reagan's Locomotives: Simple, Compound, and Electric. New Edition. 

Large 12 mo, 

Sinclair's Locomotive Engine Running and Management nmo, 

Smart's Handbook of Engineering Laboratory Practice nmo, 

Snow's Steam-boiler Practice 8vo, 

Spangler's Notes on Thermodynamics nmo, 

Valve-gears , 8vo, 

Spangler, Greene, and Marshall's Elements o Steam-engineering 8vo, 

Thomas's Steam-turbines 8vo, 

Thurston's Handbook o* Engine and Boiler Trials, and the Use of the Indi- 
cator and the Prony Brake 8vo, 

Handy Tables 8vo, 

Manual of Steam-boilers, their Eesigns, Construction, and Operation.. 8vo, 

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Thurston's Manual of the Steam-engine 2 vols., 8vo, 10 oo 

Part I. History, Structure, and Theory 8vo, 6 oo 

Part II. Design, Construction, and Operation .8vo, 6 oo 

Steam-boiler Explosions in Theory and in Practice i2mo, i 50 

Wehrenfenning's Analysis and Softening of Boiler Feed-water (Patterson) 8vo,' 4 00 

Weisbach's Heat, Steam, and Steam-engines. (Du Bois) 8vo, 5 00 

Whitham's Steam-engine Design " g vo> - OQ 

Wood's Thermodynamics, Heat Motors, and Refrigerating Machines. . .8vo,' 4 00 

MECHANICS PURE AND APPLIED. 

Church's Mechanics of Engineering g vo 

Notes and Examples in Mechanics 8vo 

Dana's Text-book of Elementary Mechanics for Colleges and Schools. .i2mo! 
Du Bois's Elementary Principles of Mechanics: 

Vol. I. Kinematics gvo 

Vol. II. Statics 8vo! 

Mechanics of Engineering. Vol. I Small 4x0! 

Vol. II. Small 4to, 

♦•Greene's Structural Mechanics g vo 

James's Kinematics of a Point and the Rational Mechanics of a Particle. 

Large i2mo, 

* Johnson's (W. W.) Theoretical Mechanics i2mo, 

Lanza's Applied Mechanics g vo 

* Martin's Text Book on Mechanics, Vol. I, Statics i2mo, 

Vol. 2, Kinematics and Kinetics . .i2mo, 
Maurer's Technical Mechanics g vo 

* Merriman's Elements of Mechanics i2mo 

Mechanics of Materials g vo 

* Michie's Elements of Analytical Mechanics gvo, 

Robinson's Principles of Mechanism gvo 

Sanborn's Mechanics Problems Large i2mo 

Schwamb and Merrill's Elements of Mechanism gvo, 

Wood's Elements of Analytical Mechanics 8vo, 

Principles of Elementary Mechanics i2mo, 

MEDICAL. 

* Abderhalden's Physiological Chemistry in Thirty Lectures. (Hall and Defren) 

8vo, 
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19 



AUG 21 1908 



Compressed Air Plant 
For Mines 



THE PRODUCTION, TRANSMISSION AND 
USE OF COMPRESSED AIR, WITH 
. SPECIAL REFERENCE TO 
MINE SERVICE 



By ROBERT PEELE 

Mining Engineer and Professor of Mining in the School of Mines, Columbia University 



8vo. xix -f- 325 pages, 112 figures. Cloth, #3.00 (12/6 net; 



NEW YORK: 

JOHN WILEY & SONS 

London: CHAPMAN & HALL, Limited 
1908 



CONTENTS 



PART FIRST 
PRODUCTION OF COMPRESSED AIR 

PAGE 

Preface, v 

List of Illustrations, xi 

CHAPTER I 

Introduction. Development of Air Compressors. Compressed Air 

versus Steam and Eleetric Transmission of Power, i 

CHAPTER II 

Structure and Operation of Compressors: Straight-Line and Duplex. 
Compound Steam End; Stage Compressors; Direct- and Belt- 
Driven or Geared Compressors. Comparison of Types. Rela- 
tion of Work Done in Air and Steam Cylinders. Proportions of 
Cylinders. Compressors Driven by Water Power, 8 

CHAPTER III 

The Compression of Air. Outline of the Theory. Operation of Air 
Compressors. Isothermal and Adiabatic Compression. Modes 
of Absorbing the Heat of Compression, 43 

CHAPTER IV 

Wet Compressors. Hydraulic-Plunger and Injection Compressors. 

Injection Apparatus. Quantity of Injection Water Required, . . 52 

CHAPTER V 

Dry Compressors. Construction of the Water-Jackets. Circulation 
of Cooling Water. Piston Clearance and its Effect on Volumetric 
Capacity. Dry versus Wet Compression. Effect of Moisture 
in the Air under Compression. Effect of Injected Water, . . 58 

vii 



Viii CONTENTS 

PAGE 

CHAPTER VI 

Compound or Stage Compressors. Theory and Operation. Single- 
and Double- Acting Stage Compressors. Construction and 
Functions of the Intercooler. Deductions from the Indicator 
Card of the Stage Compressor, 71 

CHAPTER VII 

Air Inlet Valves. Chief Requisites of . Poppet Inlet Valves: their 
Construction and Operation. "Skip-Valves" for the High- 
Pressure Cylinder of Stage Compressors. Ingersoll-Sergeant 
Piston-Inlet Valve. Johnson Valve. Humboldt Rubber Ring 
Valve. Leyner Flat Annular Valve. Arrangements for Ad- 
mitting Inlet Air to the Compressor, 91 

CHAPTER VIII 

Discharge ,or Delivery Valves. Spring-controlled Poppet Valves. 
Cataract-controlled Poppets. Riedjer Discharge Valve. Dis- 
charge Area for Air Cylinders, no 

CHAPTER IX 

Mechanically Controlled Air Valves and Valve Motions. Mechanical 
Control for Discharge Valves. Norwalk, Nordberg, Laidlaw- 
Dunn-Gordon, Allis-Chalmers, Sullivan, Riedler, and Other 
Valve Motions. Cam -controlled Inlet Valve. Sturgeon Inlet 
Valve. Piston Valves, 116 

CHAPTER X 

Performance of Air Compressors. Standards of Rating. Calculation 
of Horse-Power of Single-Cylinder and Stage Compressors. 
Mean Cylinder Pressure. Temperature of Compression. Ele- 
ments of Air Indicator Card, 133 

CHAPTER XI 

Air Receivers. Construction and Functions. Underground Re- 
ceivers. Value of Cooling in the Receiver. " Receiver After- 
Coolers," 144 

CHAPTER XII 

Speed and Pressure Regulators for Compressors. Speed Governors. 
Air Cylinder Unloaders. Modes of Regulation for Steam- and 
Belt-Driven Compressors, 150 

CHAPTER XIII 

Air Compression at Altitudes above Sea-Level. Consequent Reduc- 
tion of Volumetric Capacity of Compressor. Relation between 
Compressor Output and Barometric Pressure. Mechanically 
Controlled Inlet Valves for High Altitudes. Stage Compression 
at High Altitudes, 164 



CONTENTS IX 



PAGE 

CHAPTER XIV 



Explosions in Compressors and Receivers. Discussion of Causes 
Heat of Compression. Cylinder Temperatures and Flashing- 
Points of Lubricating Oils. Examples of Explosions. Effect 
of Leakage of Delivery Valves. Precautions for Preventing 
Explosions, , . . 171 

CHAPTER XV 

Air Compression by the Direct Action of Falling Water. Theory 
of. Taylor Hydraulic Compressor. Descriptions of Plants at 
Magog, Province of Quebec, Ainsworth, B. C, and Victoria 
Copper Mine, Mich. Results of Tests, 183 



PART SECOND 
TRANSMISSION AND USE OF COMPRESSED AIR 

CHAPTER XVI 

Conveyance of Compressed Air in Pipes. Loss of Power. Loss of 
Pressure or Head. Discharge Capacity of Piping. D'Arcy's 
Formula. Richards 's Formula for Loss of Pressure. Com- 
parison of Results of Current Formulas. Compressed-Air 
Piping. Effect of Bends in Pipe-Lines, 194 

CHAPTER XVII 

Compressed-Air Engines. General Considerations. Working at 
Full Pressure or with Partial or Complete Expansion. Ratios of 
Pressures and Temperatures due to Expansion in a Motor Cylin- 
der. Corrections for Piston Clearance, etc. Nominal and 
Actual Cut-off. Work Done in a Motor Cylinder. Volume of 
Free Air Required. Cummings "Two-Pipe" System, . . . 205 

CHAPTER XVIII 

Freezing of Moisture Deposited from Compressed Air. Causes and 
Prevention of Freezing. Influence upon Freezing of High Press- 
ures in Transmission. Deposition of Moisture by Reduction of 
Pressure. Protection of Air Piping, 220 

CHAPTER XIX 

Reheating Compressed Air. Appliances for, and Results of Reheat- 
ing. Temperatures Employed and Consumption of Fuel. 
Construction and Operation of Reheaters. Use of Reheaters for 
Underground Work. Wet and Dry Reheating 225 



X CONTENTS 

PAGE 

CHAPTER XX 

Compressed- Air Rock-Drills. General Considerations as to Efficiency. 
Consumption of Air: Normal and at Altitudes above Sea-Level. 
Factors Affecting Air Consumption: Kind of Work, Character 
of the Rock, and Physical Condition of Drill. Examples from 
Practice. Proper Air Pressure for Machine Drills. Valve 
Motions, 240 

CHAPTER XXI 

Operation of Mine Pumps by Compressed Air. Disadvantages of 
Using Ordinary Steam Pumps. Simple, Direct-Acting Pumps. 
Cylinder Dimensions of Simple Pumps. Volume of Air for 
Non-Expansive Working. Horse-Power. Regulation of Ini- 
tial Air Pressure. Prevention of Freezing of Moisture. Com- 
pressed-Air-Driven Compound Pumps: Discussion of Modes of 
Using the Air. Application of Reheating, 250 

CHAPTER XXII 

Pumping by the Direct Action of Compressed Air. Pneumatic- 
Displacement Pumps. Merrill, Latta-Martin, and Harris Dis- 
placement Pumps. Pohle Air-Lift Pump: Theory and Opera- 
tion. Tests on Air-Lift Pumps. Application for Pumping 
Slimes in South -African Mills. Lansell's Air-Lift for Pumping 
in Mine Shafts, 265 

CHAPTER XXIII 

Compressed- Air Haulage for Mines. Compressed Air versus Electric 
Locomotives. Construction and Operation of Compressed-Air 
Locomotives. Modes of Dealing with Low Cylinder Temperature. 
Calculation for Pipe-Line and Charging Stations. Charging 
Apparatus. Calculation of Motive Power. Compressors for 
Charging Pneumatic Locomotives. Detailed Examples of Com- 
pressed-Air Haulage Plants, 282 



ILLUSTRATIONS 



PAGE 



Figs, i and 2. — Laidlaw-Dunn-Gordon Straight-Line Compressor. Plan 

and Elevation, 10, 11 

Fig. 3. — Ingersoll-Rand Straight-Line Compressor, Class A-i, 13 

Figs. 4 and 5. — Laidlaw-Dunn-Gordon Duplex Compressor. Plan and Ele- 
vation, 14, 15 

Fig. 6. — King-Riedler Compound Vertical Two-Stage Compressor, ... 16 

Fig. 7. — Norwalk Compound Straight-Line, Two-Stage Compressor. Longi- 
tudinal Section, 18 

Fig. 8. — Norwalk Straight-Line, Two-Stage Compressor, with Simple Steam 

End, 19 

Figs. 9 and 10. — Leyner Straight-Line, Two-Stage Compressor. Plan and 

Elevation, * 20 

Fig. 11. — Sullivan Straight-Line, Two-Stage Compressor. Longitudinal Sec- 
tion, Inset page 22 

Fig. 12. — Sullivan Duplex, Two-Stage Compressor. Longitudinal Section 

through Low-Pressure Cylinder, 21 

Fig. 13. — Leyner Duplex, Two-Stage Compressor, with Simple Steam Cylin- 
ders, 23 

Figs. 14 and 15. — Riedler Cross-Compound Two-Stage Compressor; 15" and 

24"X36" Air Cylinders. Plan and Elevation, 24, 25 

Figs. 16 and 17. — Allis-Chalmers Cross-Compound Corliss, Two-Stage Com- 
pressor. Plan and Elevation, 26, 27 

Figs. 18 and 19. — Laidlaw-Dunn-Gordon Duplex, Cross-Compound Compres- 
sor, with Two-Stage Air Cylinders. Perspective View and General 
Plan, Elevations and Sections, Inset and page 29 

Fig. 20. — Combined Air and Steam Cards, 31 

Fig. 21. — Duplex, i6 // X3o" Risdon Compresser, Driven by 16 ft. Water-wheel, ^^ 

Figs. 22 and 23. — Water-Driven Risdon Duplex Compressor. Plan and Ele- 
vation, 34, 35 

Fig. 24. — Ingersoll-Rand Water-Driven Compressor, 37 

xi 



Xll ILLUSTRATIONS 

PAGE 

Figs. 25 and 26. — Rix Water-Driven Compressor at North Star Gold Mine, 

Calif. Side and Front Elevations, 38, 39 

Fig. 27. — Ingersoll-Sergeant Straight-Line, Belt-Driven Compressor, ... 41 

Fig. 28. — Air Compression Temperature Diagram, 47 

Fig. 29. — Air Indicator Card, 49 

Figs. 30, 31 and 32. — Air Indicator Cards, Showing Effect of Cooling, ..051 

Fig. 33. — Humboldt Wet Compressor, ....'...._..... 53 

Fig. 34. — Hanarte Wet Compressor, 54 

Fig. 35. — Air Cylinder of Nordberg Compressor, „ . . . 59 

Fig. 36. — Air Cylinder, Class E, Laidlaw-Dunn-Gordon Co., 60 

Fig. 37. — Air Card Showing Effect of Clearance, 63 

Fig. 38. — Diagram of Effect of Clearance on Capacity of Dry Compressor, . 64 

Fig. 39. — Section of Air Cylinder, Showing Method of Reducing Clearance, . 65 

Fig. 40. — Section of Piston, Johnson Compressor, 65 

Fig. 41. — Diagram of Norwalk Two-Stage Compressor, 76 

Fig. 42. — Horizontal Intercooler. Ingersoll-Rand Co., 81 

Fig. 43. — Intercooler. Sullivan Machinery Co., 82 

Fig. 44. — Leyner System of Intercooling, 85 

Fig. 45. — Vertical Intercooler. Ingersoll-Rand Co., , 87 

Fig. 46. — Combined Air Card of Two-Stage Compressor, 88 

Fig. 47. — Norwalk Poppet Inlet Valve, ... 94 

Fig. 48. — Laidlaw-Dunn-Gordon Poppet Inlet Valve, 95 

Fig. 49. — Diagram of Effect of Valve-Spring Resistance on Volumetric Capac- 
ity of Compressors, 97 

Fig. 50. — Air Card Showing Effect of Valve Resistance, 98 

Fig. 51. — "Skip-Valve." Norwalk Iron Works Co., 100 

Fig. 52. — Cylinder of Piston-Inlet Compressor. Ingersoll-Rand Co., . . 101 

Figs. 53 and 54. — Johnson Air Valves, 103 

Fig. 55. — Humboldt Rubber Ring Valves, 104 

Fig. 56. — Leyner Compressor. Part Section, Showing Flat Annular Air 

Valves, 106 

Fig. 57. — Leyner Annular Inlet Valve, 107 

Fig. 58. — Laidlaw-Dunn-Gordon Poppet Discharge Valve, in 

Fig. 59. — Norwalk Poppet Discharge Valve, 112 



ILLUSTRATIONS Xlli 

PAGE 

Fig. 60. — "Express" Poppet Valve. Riedler Compressor, . . _. . . -113 

Fig. 61. — Valve Motion of Low-Pressure Air Cylinder. Norwalk Com- 
pressor, 1 . . . ng 

Fig. 62. — Section of Air Cylinder of Nordberg Compressor, 120 

Fig. 63. — Section of Air Cylinder. Laidlaw-Dunn-Gordon Co., 121 

Fig. 64. — "Cincinnati" Valve Gear. Laidlaw-Dunn-Gordon Compressor, . 122 

Fig. 65. — Standard Air- Valve Motion. Allis-Chalmers Co., 124 

Fig. 66. — Sullivan Air Cylinder, Showing Corliss Inlet Valves, 125 

Fig. 67. — Riedler Air- Valve Motion, 127 

Fig. 68. — Details of Riedler Inlet Valve, . . 128 

Fig. 69. — Details of Riedler Discharge Valve, 129 

Fig. 70. — Cam-Controlled Inlet Valve, 130 

Fig. 71. — Sturgeon Inlet Valve, 131 

Fig. 72. — Ideal Air Card, 141 

Fig. ]$■ — Diagram. Elements of Air Indicator Card, 141 

Fig. 74. — Vertical Air Receiver. Norwalk Iron Works Co., 145 

Fig. 75. — Horizontal Receiver-Aftercooler. Ingersoll-Rand Co., . . . 146 

Fig. 76. — Clayton Governor and Pressure Regulator, 151 

Fig. 77. — Norwalk Pressure Regulator, 153 

Fig. 78. — Norwalk Pressure Regulator, 154 

Fig. 79. — Clayton Pressure Regulator, , 155 

Fig. 80. — Rand Imperial Unloader. Sectional View, 157 

Fig. 81. — Sullivan Governor and Unloader, 158 

P^ig. 82. — Ingersoll-Sergeant Regulator and Unloader, 160 

Fig. 83. — Laidlaw-Dunn-Gordon Air Governor, 161 

Fig. 84. — Air Cards Showing Results of Compression at Altitudes above 

Sea Level, 165 

Fig. 85. — Taylor Hydraulic Air Compressor, 185 

Fig. 86. — Taylor Hydraulic Air Compressor. Detail of Head-piece, . . . 186 

Figs. 87 and 88. — Hydraulic Air-Compressing Plant at Kootenay, B. C, 

Inset and page 190 

Fig. 89. — Expansion Curves of Steam and Air, 209 

Fig. 90. — Card Showing Work Done in Motor Cylinder, 213 

Fig. 91. — Leyner Compressed-Air Reheater, 231 



XIV ILLUSTRATIONS 

PAGE 

Fig. 92. — Cast-iron Coils, Leyner Reheater, ..... 232 

Fig. 93. — Sergeant Reheater, ,.. = = ,. . 233 

Fig. 94. — Rand Reheater, ....,., 234 

Fig. 95. — Sullivan Reheater, ..>....-. 235 

Fig. 96. — Merrill Pneumatic Pump, .............. 266 

Fig. 97. — Diagram of Pohle Air-Lift Pump, 271 

Fig. 98. — Diagram of Lansell's Air-Lift Pump for Mine Shafts, .... 280 

Fig. 99. — H. K. Porter Four-Wheel, Single-Tank Compressed- Air Mine Lo- 
comotive, . 286 

Fig. ico. — Small H. K. Porter Compressed-Air Locomotive, 287 

Fig. 101.— Baldwin Six-Wheel Compressed- Air Locomotive, 288 

Fig. 102. — Baldwin Four -Wheel Compressed-Air Locomotive, ..... 288 

Figs. 103, 104 and 105. — Plan, Elevations and Sections of Baldwin Com- 

pressed-Air Locomotive, 289, 290, 291 

Fig. 106. — Compressed-Air Locomotive Charging-Station, 297 

Fig. 107. — Xorwalk Locomotive Charging Compressor, 301 

Fig. 108. — Air-End of Ingersoll-Rand Three-Stage Locomotive Charger, . . 302 

Figs. 109 and no. — Low- and High-Pressure Air-Ends of .Ingersoll-Rand 

Four-Stage Compressor, 303 

Fig. in. — Perspective View of Ingeysoll-Rand Four-Stage Compressor, . . 304 

Fig. 112.— E. A. Rix Compressed-Air Locomotive for Empire Mine, Grass 

Valley, Cal., 307 



DRY COMPRESSORS 6l 

until it is finally discharged. An active circulation is thus main- 
tained. For furnishing the cooling water a tank is often provided, 
set at some elevation above the compressor, or a small pump may 
be employed. 

Naturally, a partial cooling only can be effected by water-jacket- 
ing the air cylinder. Much depends on the speed at which the com- 
pressor is run. In the best single-stage compression, to say seventy 
or seventy-five pounds, and at not over 300 feet piston speed, it is 
doubtful whether more than about one-half of the total possible 

P' / V \ n 
cooling can be effected ; that is, in the equation — = ( — j ,n would 

be equal to, say, 1.22 to 1.25. Heat- is generated faster than it can 
be abstracted, and only a portion of the volume of air passing 
through the cylinder comes into direct contact with the cooling sur- 
faces. It is important, therefore, that as much as possible of the 
total cylinder surface be covered by the jacket, and that the piston 
speed be moderate. But, in a dry compressor, as the air is com- 
paratively free from moisture, some heating is not so objectionable 
as it would be in a wet compressor. As a matter of fact, the cylin- 
der, discharge pipe, and even the receiver, are usually quite hot 
when the compressor is running at full speed; often too hot to be 
touched with the hand. In a plant at Birmingham, England, with 
well-jacketed cylinders, and compressing only to forty-five pounds, 
a temperature of the air at delivery has been observed as high as 
280° F. In this case the compressor is large, so that the super- 
ficial area of the jackets is small as compared with the volume of 
the cylinder. It is probable that the heat of compression in 
dry compressors ranges from 200° to a maximum of 400 F. for 
the ordinary pressures used in mining, though it does not often 
exceed 350 . Care should be taken not to allow the temperature 
to rise above this point.* At a large mine in Montana, the writer 
has observed the thin wrought-iron delivery pipe of a fifty-drill 
compressor red-hot for a distance of nearly six inches from the 
cylinder shell. Driving compressors at too high a speed (when not 

* T. G. Lees, Trans. Federated Inst. Mining Engs., Vol. XIV, p. 569. See 
also Chapter XIII of present volume. 



MECHANICALLY CONTROLLED VALVES AND VALVE MOTIONS 1 27 




> 



< 

2 







COMPRESSED AIR ENGINES 



213 



3. Working with Complete Expansion. In the theorerical 
card, Fig. 90, is shown the relations of the compression and ex- 
pansion lines, the shaded portion representing the useful work 
done by the complete expansion of cold air in a motor cylinder. 




Vols, in Cu. Ft. 
Fig. 90. 

When the expansion is adiabatic, the same relations exist between 
pressures, volumes, and temperatures as were set forth in the dis- 
cussion of adiabatic compression, viz: 

~pr /V\ 72 = 1.406 /T r \ « — i 

The theoretical work done by complete adiabatic expansion 
may be expressed by a formula similar to that employed for com- 
pression, but with an inversion of certain of the quantities, thus: 

W =^Z7 PV ['"(J)^]- in which 

W=the theoretical foot-pounds of work done by the expan- 



PUMPING BY THE DIRECT ACTION OF COMPRESSED AIR 267 

chamber or tank, suitable valves being provided for controlling 
the admission of air and water. As the name implies, the water 
is displaced by the air and is discharged from the tank through 
a column pipe. There may be either one or two tanks, the column 
pipe in the latter case being common to both. With one tank, the 
flow of water from the pipe is intermittent ; with two, practically 
constant, the pair of tanks then resembling in their relation to each 
other the chambers of the ordinary steam pulsometer pump. 
Aside from the simplicity of construction and absence of moving 
parts subjected to wear, which adapt it for mining, as well as for 
general service, such as pumping from wells and other sources of 
water supply, the pneumatic-displacement pump has a distinct 
advantage for pumping chemical solutions, acids, etc., which 
would corrode the mechanism of a piston pump. It is evident, 
however, that the head or pressure under which the ordinary 
displacement pumps will work is limited absolutely by the air 
pressure employed. 

The double-chamber pump, as built by the Merrill Pneumatic 
Pump Co., will serve to illustrate details of construction and 
operation. Fig. 96 is a diagram of this pump, showing the sub- 
merged chambers, with their connections to the discharge pipe. 
Air from the compressor enters a chest through an automatic 
valve, which opens connection alternately with the two water 
chambers. The air pressure to be employed depends on the 
height of lift. Since the weight of a column of water is 0.434 lb. 
per foot of head, the height to which a given air pressure will raise 
water is equal to the gauge pressure divided by 0.434 ; thus, air at 

80 
80 lbs. will pump to a height of • = 184 ft. In practice, how- 

•434 
ever, to cover friction, leakage, absorption of air by the water, 

and to provide the necessary dynamic head for overcoming inertia 
and securing a proper speed of discharge, an additional air press- 
ure is required. In terms of volume, 1 cu. ft. of water will be 
displaced per cu. ft. of compressed air. One cu. ft. of air at 80 

lbs. = = 6. ^ 3 cu. ft. free air. To this should be added 

15 



AUG 21 1908 



COMPRESSED AIR HAULAGE FOR MIXES 309 

200 lbs. Average working pressure, 180 lbs. The cost of the 
plant was as follows : 

One Nonvalk 3 -stage compressor, erected 85,180. 74 

Pipe-line, 4,200 ft., 5 in., including 3 charging stations 2,951.06 

Two Baldwin compressed-air locomotives and fittings 4,904.33 

Alterations in gangways to adapt them for locomotive haulage 665 . 1 7 

Total cost 813,701.30 

Daily operating cost, for 180 days in the year S14.69 

Fixed charges, depreciation, repairs, etc., figured at 10 per cent., together 

with cost of steam power 9 . 00 

Total running expenses per day S23 . 69 

Cost per car, at 660 cars per day .' 3.6 cents 

Previous cost of mule haulage per car 5.1 " 

Saving per year, about : Si,8oo.oo 

5. At the Wilson Colliery, of the D. & H. Coal Co., a large 
locomotive was installed by the Dickson Manufacturing Co., hav- 
ing six 26-in. drivers; wheel-base, 7 ft.; cylinders, 9 ins. X 14 ins.; 
gauge of track, 30 ins. The locomotive carries two tanks, 18 ft. 
6 ins. and 15 ft. 6 ins. X 30 ms. diameter, with a capacity of 160 cu. 
ft. of air at 600 lbs. Pipe-line, 4,100 ft. long; pressure, 700 lbs. 
Total charging time, 1 min. 25 sees. After reduction to 125 lbs. 
working pressure the air is reheated. Trains usually consist of 
30 cars, each weighing loaded, 5,850 lbs., though the locomotive 
has a capacity of 50 cars. Grades, from 9 ins. per 100 ft. against 
the load, to 12 ins. per 100 ft. in favor of the load. Round-trip 
time, for 8,200 ft. plus a switching distance of 800 ft., 16 min. 
Cost of haulage per ton-mile, gross, about 1 J cents. 

6. The Anaconda Copper Mine, Butte, Mont., is provided 
with a number of compressed-air locomotives with 5-in. Xio-in. 
cylinders and weighing 10,000 lbs. Over all dimensions: 
height, 58 ins. ; width, 58 ins. ; length, 10 ft. 4^ ins. ; four driving 
wheels, 23 ins. diam. ; wheel-base, 36 ins., designed for curves of 
12-ft. radius. Capacity of main tank, 47 cu. ft.; pressure, 550 lbs. 
working pressure, 125 lbs.; charging time, 60 sees. Length of 
haul, 2,400 ft. round trip; load, 6 cars, weighing loaded 3,45° ^ s * 




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